I agree with all the comments so far about the high caliber of individuals on this site - and as TWL says, if you can't do it here, good luck anywhere else. On the other hand, you have to be careful anywhere you go, and Audiogon is no exception. Out of self-protection, I've started a personal list of users not to deal with again after some bad experiences. The funny thing is - the two so far were both dealers (one of which is an Audiogon "featured dealer", can you imagine that?)
I have had the best experiences with private transactions - I find the people here to be honest and proactive, and to have a vested interest in their gear, the sanctity of the transaction, and their continued reputation and involvement with this site. I've also had great experiences with many of the dealers who offer items on this site. There do remain a few dealers who are so jaded however, that customer service and even basic courtesy are out the window. These are people to stay away from. I suggest calling any dealer and talking to them on the phone for 10 minutes (minimum) before dealing with them. Anyone with similar experience will see the built-in irony in that test.
No classified marketplace online or offline does background checks on individuals - the best thing to do is to deal with people with ample good feedback, and never be afraid to bail out if something is uncomfortable. I've saved myself a couple times by trusting my gut.