I sell some leftover documentary footage on Shutterstock, mainly nature shots like landscapes and such. I also tried to register to istockphoto couple of years ago but they had a very awkward registration process which required samples of the types of clips I would like to sell via them, and the service rejected literally all my sample clips saying that "we already have this type of material" ("not interested about your stuff") which was very frustrating because the idea of the samples was to just evaluate the technical quality of my footage, not the content/subject matter Shutterstock had better royalties percentage at the time so I just started to use their service and did not bother with the istockphoto
they may have changed their service though because this was couple of years ago so you may see if they have made it easier.
I think the Business Practices -forum is about right for this type of thread.
Shutterstock has been very good for my needs and it was very easy to start and to add large amounts of clips per batch.
If you want to create material specifically for stock footage use then you need to investigate more about which type of footage sell best ( high quality aerial and slow mo shots for example) , you need to sell a clip multiple times if you want to specifically create stock footage from the start and not to just dump the leftover material from various completed projects to the stock