Another way to get a little boost from eBay purchases is to sign up with MyPoints. Click through the MyPoints site every time you go to eBay, and you'll get points for purchases. Points can be redeemed for gift cards to other merchants, etc. MP is one of the better regarded rewards programs, has been around a long time (there's a site called CompareRewards that will back me up on this). It may not be the only reward site that "pays" for eBay purchases, but I think it is the premier site in terms of reliability and point value.

BTW there is a referral program but soliciting is probably against forum rules