The customers who use stock photos are, advertising agencies, printers, web designers, publishers, craft, writers etc. flowers and macros sell but in the minority, what sells in the majority are posed shots with models at say a dentist, shopping centre, market, home, etc. then food shots.
most stock agencies allow you to view their offerings so would be a good idea to go and have a look at whats on offer,
http://www.istockphoto.com/browse/latest/ also don't assume just because you take a photo it will be on sale, most inspectors are brutal photographers and agencies only want the best of your work, this means rarely getting anything accepted higher than ISO 100, and size is also a factor. And key wording your images is also an important factor
My sole reason for getting into stock was to see if I was a good enough photographer to get my images accepted, and then to see if anybody would like them enough to pay money for them, in both accounts I am now satisfied, the money I don't need, but I do get a buzz knowing there are publications, books and magazines that have my photos in them.
David