Pay close attention to the seller's feedback rating on any item you are considering bidding on.  DON'T hesitate to read the negatives if there are any to get an idea of what the seller is like to do business with.  Granted seller's cannot satisfy everyone all the time but, based on personal experience, if the feedback for the seller is rated below 99.0%, I take the time to see why. 

Also, if you have a good experience with an e-bay transaction - be sure to leave positive feedback for the seller as it builds their credibility.

As a seller also, feedback for buyers is important to me.  Bidders with feedback of 1 or less, or -1, are not allowed to bid on my auctions without prior approval via e-mail.  These blocks can be activated when listing your item.  You can also block unsavory bidders from bidding on your auctions, especially if you have had a bad experience with them.