save for overtly hacked accounts, substantial amounts of positive seller feedback on sites like audiogon or audiomart is very important... it speaks to the person doing the trading, their character, their level of attentiveness and care
then, to be totally sure, ask to speak to the person, get comfortable, if you still have some lingering doubts
ultimately, for buying all used, valuable items, it is important to get a good feel for the seller/prior owner, understand what kind of people they are
all this said, buying used gear is just well suited for some people -- for this minority of folks, dealers or direct sales is readily available