I’ve got some up on iStockPhoto but I don’t get many sales. I actually prefer uploading them on places like Flickr and deviantart where people can view them for free. People charge way too much for stock photos online anyway.
I’ve never thought about putting any of my images up for sale. I had a quick look through some of the sites you mentioned there and as biggles pointed out, some of the images are really expensive. I couldn’t see myself buying any of them for that price.
I tried it once, but I only a got a few sales. The problem might have been that I never uploaded my best work, since I prefer to keep those for myself, maybe handing out a few prints to friends and family.