I've been pleased with both
Sportlots.com and
Checkoutmycards.com. I've bought and sold on both.
Sportlots is more work on the part of the seller, as you have to do all your own shipping and inventory control, but is great for unloading commons.
Check Out My Cards is consignment, so depending on how much you're willing to pay per card for faster processing into their system, seems to work better for higher end cards and visually is a much nicer system.
Ultimately, both end up being cheaper to deal with than eBay, but of course, you end up with less traffic, though both sites get a fair bit.