Pixels and Fine Art America are mirror sites. Whatever you upload to one shows up on the other. The only real difference is that Pixels sells more merchandise items like cell phone cases that don't show up on FAA. Someone put it nicely recently when they said something to the effect that FAA is the art gallery and Pixels is the gift shop.
By the way, whatever you upload to Pixels or FAA also shows up automatically on your personal Artist Website if you're a Premium member.
EDIT: There's also a subdomain of Pixels called licensing.pixels.com where you can sell digital downloads with publication licenses. They are made available for licensing IF you enter licensing prices when you upload to FAA.