ArtInfo has profiled five "Award-Worthy Web Galleries" that allow you to buy and sell work online. It's a very mixed bunch, aiming at very different buyers/sellers, but they all seem worthy enough to mention.
PicassoMio gets the "Best of the Behemoths" award, which has more than 20,000 works by 2000 different artists worldwide.
WeHeartPrints wins the "Labor of Love" award, as it's small, focused, and looked after by one person.
MixedGreens is awarded the "Best of Both Worlds" prize as it is both a bricks & mortar gallery and a gallery selling works online (I'm sure all bricks & mortar art galleries will eventually move to selling work online too.. even if they only offer small works online by their exhibiting artists.)
AltPhotos gets the "Three Cheers for Democracy" award, as it's 12,000+ members are allowed to interact, comment, and sell their photos online.
Lumas wins the "Finest in Photos" award as it offers a great range of limited edition fine art photography.