Anecdotal evidence has taught me that not every new lens returned to a store can be labeled a subpar sample nor that used lenses sold are being put on the market, simply in that they are inferior to another sample of the same lens (other than the seller would like to fund some other piece of equipment). I believe there are way too many factors to make generlizations.
I once ordered a highly touted "new" 3rd party lens from a very well know reputable mail order/brick & mortar store. What I received was a lens that by anyone's standards was "quite heavily" handled much like a store demo and even looked used...fingerprints over the entire lens (who's finish was known to affix dust and fingerprints), plastic bag stuffed below cardboard packaging, which had also been mangled. Lets say it was a sample that wouldn't even pass the E+ cosmetic rating of a new lens. I needed it for a weekend endeavor, so I had no choice but to use it. In all respects it was perfect, optically and mechanically...most notably optically. Subsequenty over time I had a chance to extensively compare this sample against 3 other samples of the same lens and in all honesty, performance between mine and the other 3 wasn't even close, as my initial copy performed head and shoulders above the other 3.....especially in the critical range it was designed for. The other 3 performed similarly to each other, except for minor differences. Go figure!
Alternatively, sometimes testing multiple samples of a given used lens is necessary for certain applications, even when all the samples may be well within specs of what the lens was designed for. For example, I was shooting Pentax 645 film at the time I aquired the 645D digital body. Similar "exceptional" copies of the same lens that perfomed nearly identical on the 645 film body and obviously would have been excellent samples to use on film (and were all designed and manufactured and tested in the film era), unfortunately didn't perform well on the 645D. Often times only one of these samples was adaquate when used in conjunction with a digital sensor. This was the case with many of their lenses (even when critical Af fine tune was determined for each on the 645D). So with certain systems, often times a lens or a given sample of a lens designed in the film era can be well within specs in terms of performance with film, but not be up to par, when used on a digital body.
I know I've gotten somewhat off topic with regards to this thread. With bodies that are returned but sold as new, this is often different than a returned lens in some respects. As with my example cited above, where the product one receives may not be "new" as described, if it's performing flawlessly, then I would think at the very least the buyer should be given the option to either 1) exchange for an un-opened one, 2) some mutually agreeable compensation or 3) returned for a refund. Tough call to make.
Dave (D&A)