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USED GEAR PURCHASES:

My experience is based on a lot of horse trading in our retail outlet in which we dealt heavily in used camera gear and repaired a lot of used equipment. Every trade we took in represented a part of the profit structure of the transaction that occurred with the sale of a new part and we wanted to earn back that share by selling the used trade.

FACTS ABOUT QUALITY
Generally there is little difference in lens glass till you get to the OEM or better lines. Then the science gets involved. In the old days few radical technologies existed that were only gifted to one manufacturer.  Today, many of the same qualities, features and benefits are shared by the better manufacturers, as they buy from the same vendors, only they were given different names.  

Major and obvious differences involved proprietary lens mounts connections and coupling devices, the main reason for these differences was the patent bureau and licensed rights to protect copy by third party manufacturers which failed when SIGMA and Dr. Yanachi reverse engineered the codes.

HINTS ON WHAT TO BUY:

There are signs that you can look for to determine the respective value. Value is really three things:
  •  What you think it's worth
  •  What the seller thinks it's worth
  •  What it is really worth.  Sounds like the used car business…it is.

PICTURES
First off, most listings will include photos of the listed lens. I would be very skeptical in bidding on a listing that didn't include a photo (or several) of the listed item. Carefully examine the photos in the listing. You can usually get a good idea of the overall cosmetic condition this way. And make sure you can see the serial number.
No photos, ask for some or pass. Occasionally, a seller may put the item "best face" forward, so consider that all defects may not be shown. Ever buy steak at the local A&P or Publix, only to discover that great steak had another side to it, the grizzle side. That's why packages for meat are cardboard and plastic food wrap.  The cardboard absorbs any blood and the plastic wrap shows the good side. 

A good seller will always provide a photo of the notable flaws and describe them  in the item description. Check your seller's feedback for any previous problems pertaining to "items not as described".

DESCRIPTION
Read the listing description. The more detail provided by the seller on the overall condition of the item, the better. Be skeptical of a short, glowing A+ description coupled with a fuzzy cell phone picture of the item. The item may in fact be a beauty, but it would be difficult to confirm before the item arrives and you can inspect it. 

If it isn't, you will likely experience the Ebay dispute process firsthand. Be wary of the term "Mint" when a seller describes item condition. If the item is "Used", expect that it will show (even the lightest) signs of use, and cannot therefore be described as "Mint" which implies "unmarked".

Unless the following detailed information is clearly provided in the listing, the buyer should contact the seller to clarify the following issues:
1-The condition of the lens optics (glass), both the exposed surfaces and internal components.
2-The condition and function of the lens aperture.
3-The condition of the mount.
4-The condition and action of the focus ring (and zoom ring if applicable)
5-The overall cosmetics of the body of the lens or any other notable issue the seller may feel is worth mentioning like mold, disease and the dog ate it.    

GLASS:
The glass can have numerous problems. Most easily detected are scratches on the exposed glass elements. Sometimes, lighter scratches caused by poor cleaning technique can be present and are more difficult to detect. Bright light is usually sufficient to see "cleaning marks": 

Examine the lens with the light reflecting off surfaces at several angles and you should be able to tell if any scratches are present.Lens elements are generally "multicoated" with layers of non-reflective optical material to minimize light reflection and the resulting lens flare and ghosting associated with the multiple reflective surfaces of lenses.

Blemishes or areas in the multi-coating where material has been smeared or removed by a bump to the glass or a solvent splash are less critical flaws provided they are very small (1 mm diameter). A small blemish shouldn't affect image reproduction. But they should be noted in the listing if present.

A lens that has been stored in a moist or humid environment, or stored after getting wet can have a fungus bloom inside the lens. Fuzzy spots and mycelial filaments are both bad news. The fungus can secrete an acid that etches the multi-coating of the elements, so even if you can disassemble and clean the lens, the damage is likely permanent and will likely affect the sharpness of the lens' image reproduction.

Examine the lens by peering through it from the lens mount side and look at a bright surface (not the Sun!). Open the aperture to ensure you get a good look. (Read on for information on how to do this in the "The Aperture" section). If there is anything visible inside the lens, this is not good news. The light path inside the lens should be completely clear of any opacities.Occasionally, a few tiny dust particles may be noticed when looking into the front of a lens. Especially in older, larger lenses, this is normal and shouldn't have an affect on lens performance. 

Any dust should however be described in the listing. Looking into the front of the lens in bright sunlight will expose the presence of any dust particles.(Don't look through the lens at the sun, but rather, use the sunlight to illuminate the lens interior as you would use a lamp to read a book.)

Using the sun or a bright light source, deep inside the front of the lens you should also be able to see the lens aperture or "iris" - the circular opening that controls the amount of light that passes through the lens. It should be grey/black and a uniform matte-like in apearance, not wet or shiny looking. If it is, you may have oil on your aperture blades. Below, I have detailed another check to confirm if this is the case.

THE APERTURE:
When looking at the lens mount, you should be able to find a small metal tab located in a narrow 1" curved slot near the gold contacts, between the rear element and the stainless outer ring of the mount. This is the Aperture Control Rod. With a toothpick, you can slide the tab along its track and it should snap back quickly when released. 

You should notice the movement of the aperture blades as you do this. If the blades do not return to the closed, pinhole position,or are sluggish, you likely have an oily aperture. This check would confirm that fact if you have already visualized oil on the blades during your inspection of the front elements.

A lens that has been subject to extreme heat can develop an oily aperture. Grease from the focus gear liquifies in the heat and works its way centrally into the aperture area where it wreaks havoc. This is a moderately expensive camera shop repair that requires complete disassembly and cleaning, and can double (or more) the cost of your ebay bargain. (Never leave your camera gear in a car in the sun!)

THE MOUNT:
The mount should be clean, free of any gouging or groves, and the gold contacts should be clean and unworn.

FOCUS AND ZOOM RING:
The focus ring should turn smoothly, (no "bumps" or grinding sounds) with a bit of damping so as to not turn too easily and "creep" when not touched. Ditto the zoom, although older lenses will be a little looser due to use, and older push-pull zooms will likely creep in or out when pointed straight up or down. Any deviation from normal function should be noted in the listing.

BODY COSMETICS:
The overall cosmetics of the body should be as described in the listing. Any major dings or scratches should have been detailed in the listing, and/or photos.

Finally, when looking at an ebay lens, factor in whether the lens caps and hood are included in the auction. Caps cost about $5 each, and a dedicated hood can cost $10-15 (all plus shipping on ebay if they aren't available at a local camera store).

THE SELLER:
A competent seller should have the items' condition and what's included or not included accurately described in the listing. When you contact a seller for additional information, you should receive an accurate reply that answers your question(s). Be wary of the seller who replies "I don't know much about cameras, but the item looks good to me". The seller may be in fact truthful, but they may be missing important details about their item, or they may be off loading their junk in an "AS IS - No Refunds" auction, leaving you with a big Ebay headache and lots of paperwork and running around collecting appraisals to meet Paypal/Ebay criteria for a Dispute refund.

Upon receipt of a purchased/won ebay lens, you, the buyer should closely inspect the lens to assess if its condition is "as described", using this guide as a reference.

It's always a good idea to contact the seller immediately upon item receipt if you feel you are disappointed with the transaction in some way. A responsive, ethical seller will try to work with you to resolve any issues. You have up to 45 days to file a dispute with ebay if you cannot personally resolve your differences, but it is better to get the ball rolling sooner than later.

After doing all your research, if you are satisfied that the lens in an auction is in a condition acceptable to you, then it's time to get bidding! Good luck!

AFTERMARKET LENSES:
Dr. Yamaki of SIGMA took this to new heights with his reverse engineered line of lenses that fit most prime manufactures mounts without paying a dime of license fees.  Just recently he was awarded the induction to the PMA Hall of Fame in the camera industry for bringing low cost alternatives to the public. No mention was made of what some termed “stealing ideas’.  

In another headline his brother Yoshio Yamacki was accused of embezzling almost 7,000,000 dollars from SIGMA USA, his own brother. Some things run in families.

A complaint was unsealed this morning in federal court in Brooklyn charging YOSHIO YAMAKI, the former president of the Ronkonkoma, New York-based Sigma Corporation of America (“Sigma USA”), for his participation in an invoice fraud and embezzlement scheme involving the theft of at least $7.3 million dollars from his former employer

According to the complaint, YAMAKI was employed as the President of Sigma USA from March 1985 until January 10, 2005, during which time he also served on the Board of Directors of the Sigma Corporation. YAMAKI’s responsibilities included overseeing Sigma USA’s operations and sales, as well as balancing the corporate books, writing and signing checks, and managing Sigma USA’s accounts payable and receivable.

More on LENSES:
A Prime Lens is Sharper than a Zoom Lens…When you select a lens for your camera, your choice is evident sharpness or convenience. If you want the sharpness, don't buy a zoom lens. But, if you want convenience buy a zoom.

Why are primes sharper? Simple: Less parts ( elements).  Prime lenses usually have larger apertures than zooms and Zooms have more parts, more adjustments and harder to manufacture.  In a zoom lens the light has to be manipulated more and as a result might not be in perfect alignment at all settings.  This alone can reduce contrast and increase flare.

There are techniques for sharpness. 
Common sense for one thing!  Good lenses are the key to good quality shots. You pay for what you get, if you know what you are getting. Lens makers have mastered the word HYPE.  Get A GOOD heavy tripod, I repeat, the plastic Velbon's don’t cut it.  Use a Cable release or the self timer if need be. Image Stabilization Or Nikon VR do work but they cost. Use the lens' optimum aperture, It’s usually one or two stops down from the top. For years it was F8 and F11 and shoot.  Quality Processing.