One of the first time vendors at Photoshop World in D.C. was Deposit Photos. This stock site features both photo and vector files of a very good quality. I was lucky enough to get a trial membership and took great advantage of that, searching for and finding lots of images that I used in tutorials and as graphics for my blog. I was extremely happy with the selection of photos and their quality.
I have not registered as a seller, both of course they offer that option as well.
Be sure to check it out