I sell motorcycles & ATV's on ebay and hear about a lot of scams.  The one I see most is aimed at people who bid on the item but do not win.  The bidders will receive a e-mail that looks like a second chance offer from ebay.  In the e-mail it will say something like the high bidder fell through and will offer you the bike for a lower price.  Please if you ever get these just forward it to spoof at ebay they will tell you if it is a real offer.  The other thing if possible is to pay for the bike once you see it in person, if not possible at least do some research and talk to the person on the phone.  

One more scam is you will get a e-mail stating that you bid on a item or sold a item to someone who has already sent you payment and they are wondering when you are going to ship it.  You know that you did not do this but they will put a item number link in the e-mail.  Once you click the link it looks like you have to sign into ebay to view it (look at the HTML).  This is a fake ebay site and they just want your screen name and password. 

Ebay is a excellent place to find deals and there are so many good honest people out there.  It just takes a few to ruin the experience for everyone.  Have fun!