Always search the site in google to see if it has actually been on the web long enough to confirm the sellers claims. 

Watch for persons saying to run certain search results in certain search engines, many times these are keywords of no value or just the only ones that they could get results from.  Only pay attention to this if the keywords are actually good ones!

Check on google to find a "whois" search; this is another great way to find out more information about who registed the sites domain and how long is left on it before it expires.

Look for persons who can back up there statements with cold hard proof, if the seller gives you any flack about asking for this, almost undoubtably they are a bogus seller or scam artist.

Just some quick tips, hope you enjoyed