Ebay has a lot of nooks and crannies that are not readily apparent to the casual buyer, but help a lot when you want to get a great deal. Search is one example. For example, many people don't notice the search options on the left side of the search page. I would suggest always clicking the "all items including store inventory" check box, because it allows you to see all the deals on ebay. Also, limiting one's search to items ending in the next hour and clicking lowest price first is a good way to snipe(that is get in a bid at the last minute) an item that you may have otherwise overlooked. Be careful as some merchants may flood the market with items you aren't looking for.
To avoid this, you may want to make a price limit. For example, if I am searching for an ipod, I may limit the price to above $50 to get most of the accessories out of my search. I may also limit my search by removing certain words such as -case -charger so that those words aren't included in my search.
Also, be sure of what format you want. There are buy it now and auction tabs on the top that allow you to search only auctions or only buy it nows. If you can not find the item you want in search, use want it now.Post an ad with a descriptive title and details of what you want. The link is on the home page.
To get the most out of transactions with sellers, always remember to politely email before the end of the auction with any questions you may have. Read the auction carefully, making sure you understand the terms. Always inform your seller of what method of payment you are going to use. Be polite and friendly, and most transactions will go well.