Ebay has filters to display sold listings, so when you're looking to buy something, check out the closed auctions and see what they're going for. If the prices are similar, you might as well buy from fixed-price listings, but if auction prices are lower, you can save a lot by being patient.

You can also save searches for a fixed-priced listing below a specified value, and enable alerts, so that if someone lists something that's priced to sell, you can get it quickly.