I've purchased several items off Ebay, and have never been ripped off. Stick with a reputable person on there with a lot of good feedback, and you should be relatively safe. Going with something like a Cobra intake, you probably wouldn't lose any down low, but have an increase across the board. It would be pretty limited due to the stock heads, but definitely worth somewhere in the 20 HP range. As long as you avoid the higher RPM intakes like the Edelbrock RPM/Typhoon, you should see a fairly good increase, that would be noticable while driving.