I tried Saatchi online, it was easy because I was already a member, but only for a couple of days.

When I sold my first print I was dismayed to discover that postage, $40 (outside the US because they print them all there [something they hope to change in future]), was more than the cost of the print itself.

I stopped selling there after that, it just seemed too much to expect people to pay.

Other than that, as long as you're willing to fill in the tax thing they want for you to get your "profit", i's all simple and upfront.