Obviously, feedback is the first stop. Aside from the obvious posiitives vs negatives, the ratio of buys to sells can hint at whether someone is an enthusiast or a mercenary reseller.
I always read as many of the threads as I possibly can, preferably all of them.
Then, I Google their name, nickname, zip code and/or email address in as many possible permutations with anything else I can mine from their threads.
It then becomes pretty clear if someone might be
fraudulent or unethical (the worst possible counterparty)
a jaded, cynical, mercenary professional trying to minimize time and maximize dollars in every transaction
(almost as bad)
or
a thoughtful, considerate, enthusiastic audiophile who may only be selling to raise cash or upgrade (the best possible situation.)
For those of you who may be the first two categories, I will also ask WHY ARE YOU SELLING?
I know I know I know what does any of this have to do with anything?