I expect members here from the US already know about this but I thought I'd post just in case... or maybe for new collectors.

Ebates.com will (generally speaking) give you a discount on your eBay purchases. Often it is 1% for books but occasionally that will change (it's been at 4% the past couple months and 5% before that). It's not much but if you buy a lot or something expensive it can really add up. It's free and a pretty simple way to save some money for more books!