When in doubt about a member who has become a potential seller of an item you're interested in, use AudiogoN's many features to check him/her out before taking it further. Do a "Member Lookup" to see if this person has ever initiated or answered any threads (Thread and Answers), or has posted Reviews or his/her System, etc.
Next, look quickly at the feedback of those who have provided feedback as a gague of their reliabliity. I suppose it's possible that somsone could create multiple accounts and false feedback, but they would have to develop multiple personas as well via the discussion forums to pull it off effectively. After a while, you get a good sense of who the regular A'gon members are, and I'm always comforted to see familiar monikers in my seller's feedback section, especially if I've dealt with some of the same members personally. If you do recognize a moniker, write to that person about your concerns. Most A'gon members would not be bothered by this at all. I've recently received an email from a member who was thinking about buying an amp. He noticed that I had provided feedback for his potential seller and knew me from a previous transaction that we had entered into. It took me 20 seconds to say, ya Brad's a great guy - no concerns there.
If you are still in doubt, send a detailed email to your seller concerning issues about the product that he/she is selling, but do not tip off your suspicions of the seller's legitimacy because 1) 99+% of all A'gon members are legitimate in my experience, and 2) if you do run across a fraudulent seller, your email exchange should be as drawn out as possible so that you can collect clues as to this person's identity, location, etc. This will allow you to alert other A'gon members and staff once you're established proof of the fraud.
Learn what you need to know about the seller through an email exchange, and then call the seller to get further comfort if need be. On transactions of $1,000 or more (or you can pick the threashhold) you should make it a point to establish phone contact for the mutual assurance of both parties.
As with many A'gon members, my experiences have been all good, and this has allowed me to a build system (several systems actually) that would have cost twice the price at retail.