Do your homework before you buy! Check out everything you can about the seller. Google him, his address, email address(s), telephone #, etc. Also Google the domain if there is anything reported, it will usually be somewhere on the net.
Check the whois record to see if there have been recent changes, if there has, ask the seller where and when they purchased the domain. Ask them to provide details of the previous owner and check the whois history to see if they match.
If needs be, contact the previous owner to confirm that the domain was sold.