Wet Spot from the Minnesota Rollergirls has started a great Craigslits-style site for derby guy. DERBUY has wheels, plates, skates, hardware, and pads, as well as apparel and art. Seriously great spot to buy or sell skates.
FYI, use caution when buying pre-owned helmets. If you take one hard fall in your helmet, you should technically replace it (ha ha), so while you may be saving money buying a used helmet, you might be spending thousands of dollars in head injuries.