I like Imagekind and I know they offer museum quality prints and they also offer cards and even prints on canvas however their image search doesn't work very well so having your artwork found at random isn't that likely. To succeed there and at Redbubble (which also offers T-shirts and Calendars), you need to promote your own artwork and refer people to make the purchase.
White Iris. by Christopher Johnson
This artwork is one of my first photos from 2007! You never know what will sell. You can see I wasn't that bad.
White Geraniums 4 by Christopher Johnson
This one is from a long mixed color series I created in 2010.
Single White Oleander 1 by Christopher Johnson
This one is also from 2010 and also from a long series although most of them were pink.
I have had zero sales so far at Redbubble, but I started uploading there about a year later and I haven't uploaded everything because they require you to upload works one by one.
I make somewhat regular sales at Zazzle, but those sales are normally for cheap products so you need several sales to make a minimum payment. If you are interested in selling your artwork or graphic design online, I'd suggest uploading your work to several sites and see which work for you and be patient if you don't have the money to drive traffic to the sales site. No matter what, do NOT close a free shop or remove your work just because you don't have any sales at first. It isn't like a physical gallery where space is limited!