One must realize that there is a certain amount of trust required between parties in such a transaction. Even if the bike is right in front of you, you must decide whether or not to accept any info given you in addition to that which your experienced eye offers. If you really cannot get that far into it, you may well be better off not making any such purchase. In the end, every purchase, negotiated contract, agreement of any kind even if offered with written guarantees involves a moment of trust. Only obviously antagonistic transactions like takeovers, et. Seem the exception. Paranoia strikes every ten seconds, so they say. You either trust or avoid the seller in the best of circumstances. As George H. points out, even experts will disagree when only low to medium quality photo evidence is considered, and given the advent of things like Photoshop, one certainly hopes to develop or not develop an assessment of the seller's character for the purposes of the deal.
In other words there is nearly always an element of uncertainty present. If you can't stand this, you really need to avoid such situations if you want to be happy. We've just about all made some good and some not so good decisions in these circumstances.
Lansing, MI USA