I think it really depends on the reputation of the seller and whether there is any trial period.
For example, if someone here tells me that there is a small defect in the lens, but it has no noticeable effect on the image, I'm likely to believe them. And if I get the right to examine and return it if dissatisfied, then I would most likely take the chance. This is a small tight knit community, and nobody want the reputation of being a cheat..
OTOH, if someone on evilBay says it's mint out of the box, but "I don't know nuthin about cameras man" then I just assume it got tested, doesn't work, and they're playing stupid. Or, in the lens example "It such a small defect I couldn't get a picture of it" then it may very well have a knife gouge across the front.
So, yes, I might very well by a less than perfect lens, so long as it's disclosed and I'm not paying a premium price. In my case, the limiting factor is usually the photographer, not the lens.