First of all, i just want to say that cold air intakes are basically just a tube and a filter. However, as simple as they may seem, you can still get a crappy @** cold air intake. Going with a good brandname such as AEM, Injen, DC Sports, etc. would ensure that your tube has as little air resistance on the inside as possible, and for the filter I would either go with K&N or AEM.So those intakes you see on ebay are better than your stock intake, but they probably won't work as well as a brand name intake. Simple as that.