[myself248]

I've found a reputable local shop that I trust. In addition to a lot of new stuff, they sell used gear (possibly on consignment?), which means they've got some experts who'll do valuation on it. They have a largely professional clientele and a reputation to uphold, so I think they're trustworthy.

I'm sure I could take a lens to them and have it checked out for a few bucks, or have them broker a sale between me and another private party. Knowing what I'm getting has its own value, and I'm not opposed to paying someone for their time and expertise.

tl;dr: I don't, personally, know that. I trust someone else to know that.

[Godel_unicode]

So it sounds like we agree then; the second hand market can be quite dodgy and you need expert help to be sure you're not being ripped off.