Here are some of the most popular scams on eBay.
People use other accounts, possibly a friend's account, to bid up their auctions. Look on feedback for people who seem to win lots of items, or who have lots of withdrawn bids.
People bid items up really high to scare off other bidders, then they retract the bid in the final seconds. Look out for bid retractions.
Switch and Return
Bidders will buy something and then return it. However they send an item that isn't the original, but their own old or broken item.
Fakes and Replicas
This is very common. Watches, clothes, sunglasses, even cars. Make sure there is proof it is real, ie certificate of authenticity. Look out for comments like 'I think it is real', or 'I can't be sure'.
I hope this helps. Please rate it below.