Although Ebay has incredibly cheap parts, personally, I would stay away from them. I've seen headers crack at welds and intakes which don't fit on Ebay. Just because parts are cheap doesn't mean they are the best. Infact, sometimes, cheaper products are worse for a car.
This is just my opinion though, for great parts, I think you really pay for quality. Thats why good parts are never "cheap".
Regards,
Jason @ AJR