If you're looking to sell a part whose value you don't know because of its age or because it's rare, looking it up on eBay will give tell you what it's worth. The best function for this is to view the completed listings. You must be a registered user to use this feature, but it's free. The completed listings show what items sold and the price it sold for, the bid activity, and what didn't sell. These numbers can help you determine a realistic price for your item, or decide if it's worth selling at all.
On the buyers side, you can get an idea of how much an item costs, and how often they come up for sale. If the bidding war is taking you to a price that's uncomfortable, you can know if you need to stick with it or wait for the next one to hit the auction block.