I've been screwed once by a new, one-time Swap Meet seller who was flaky from the start. The moderator excused his sketchy listing, after I reported it. Another member then purchased the item for me.

The item was not what was advertised (wrong year radio for wrong brand of truck). The seller deleted all PMs and did not respond to email. The seller hit-and-ran - he deleted all posts and has not been seen since that first sale/posting.

Let the buyer beware.
Get all transaction information/promises in email.
Only send payment by check and to a proper address (not a PO Box).