The plain and simple truth is that you don't know!
Prior to making the purchase, I would suggest that you do your homework on the seller. Do a search on the forum they are selling on and you'll get a good idea of what the seller's reputation is. Beware of a seller with single digit posts that is trying to sell a particular higher priced item that cannot give you references.
There are allot of members that are new to a particular forum that they are trying to sell on, but have a great reputation on other forums.
It still all boils down to taking a man at his word.