One valuable feature on ebay that isn't so obvious is searching COMPLETED auctions.  It is a key feature that has benefits to both buyers and sellers.

Lets say for example, you are a buyer looking for a 1914-D wheat penny... you have been outbid on one that is closing soon and you're afraid that if you bid again you might be paying too much.  A wise decision before making your next bid is to find out what they have recently sold for.  All you need to do is go to the search tab and type in '1914-D wheat cent'. By default it will bring up all current listings. On this result page, on the left-hand side, under 'Search Options', click the box next to 'Completed listings', then click 'Show Items'.  You can then see what people have been paying for them.  If you see they are selling for less, then hold out for a different auction.

As a seller, you can use the same information to set your starting bid  or 'Buy it Now' price to maximize your profit!