To me one of the telling points I've seen several times on evil-bay is such as "Leica M-4 SLR". Anyone who would call an M an SLR certainly is NOT a camera expert!! But that admission seems to crop up all the time, especially for brokers who are selling for estates.
I think one of the best pieces of advice regarding e-b is to read the feedback from previous buyers. I've seen it said that anything under 97% positive is a questionable deal. Check out how many responses there are and how they stack up. If there are 3-400 or so responses and 98% positive, the seller should be reliable. But when you're buying used, youcan't expect guarantees.