I belong to BoundlessGallery.com. I love the artists, the art, and the community. All very friendly and willing to share professional information, tips, etc. The staff is great too and provides timely responses to any questions, concerns, etc. the artists have or are in need of. And you can join at one of 3 membership levels to afford artists a choice for the commission amount Boundless receives (the highest commission is 10%.)
www.Minigallreyworld.com is a UK based site that is rather nice. It gets lots of traffic however you have to pay to be on it. Like most sites the less expensive stuff sells the best but it is good if you want to have a presence in the UK market. Let me know what you think of it if you pay the site a visit.
As a resource for artists I really like www.painterskeys.com from Canada. You sign up for a free twice weekly email which is usually quite interesting but the best part is the click backs where artists from all over world reply to the topic.
Have any of you sold any work as a result of any of the web sites you have posted above? Where do the buyers find work? Community is great, cash rocks! LOL