Buyers commonly make the mistake of never checking out what else a Seller has on offer. Many larger Sellers (Power, Platinum, over 500 feedbackers), have multiple same items up for Auction (usually few hours apart) AS WELL AS the same item in a "Buy it Now" sale. Make sure you look for the latter, as this price is where the Seller is making a comfortable profit on the item. NEVER, I repeat never, bid higher than this amount in an Auction for the same item. It certainly amazes me that many people do bid higher than the "Buy it Now" price freely available from the same Seller.
Also make sure you use the Search function, because invariably you'll find the same item from another seller both at Auction and "Buy it Now", which is cheaper still. Place them in your Watch Item list, and if you don't need the item urgently, watch a few Auctions go to completion and see what price people are willing to pay. Most sellers will have more of the same item available over the next few days or even weeks. Generally the final price tends to fall as the days proceed, and more buyers are "taken out of the race"
Always take into consideration the cost of postage & handling. It can vary widely from Seller to Seller. Remember it is the total cost that is important to the Buyer, not just the final bid price.
Hope it helps. Remember DONT OVERBID. It makes it harder for those looking for a bargain. Good Luck and good eBaying.