Really depends on which parts you're referring to because people sell all kinds of stuff on ebay, some of it junk or even fraudulent, some of it legitimate. For the most part "ebay parts" are an unknown quantity. If you want to save a buck you could try some of those cheap parts. If you are willing to spend more go for the name brand parts. With brand name parts, many have been independently reviewed and dyno tested (if applicable), and the company has a reputation in the community, so if you're looking for greater peace of mind this is the way to go. With cheap parts the best you can hope for is that someone on the forums has already purchased and installed the part, so you have something on which to base a decision. But watch out for those "seat of the pants" impressions because they can be wildly inaccurate.