Be Careful and do your research on bike parts. : eBay Guides

You need to shop the internet and be informed on what name brand companies are selling there product for. Find out the specs and fit of the product you are considering to bid on. On new products ask about warranty and installation. There are several very good forums you can search that have experienced members who would be more than willing to help you out. Education about the product is very important, don't simply believe what you read in an auction. Sellers want to sell after all don't they? Also be very clear on shipping charges before you seal the deal, you would be surprized how it can add to the sale cost.