I'm one of the picky buyers of stock photos. There are just sooooooo many to choose from!
I buy photos for my online training courses. For me, the photos need to "support" the training topic/content. For example, I might search for someone at the cash register or someone on the phone for a customer service course. Or I might search for "team" for new employee training. It's pretty random, really, but I generally have something vaguely in mind when I do a search, and generally speaking, the photos need to have people in them (for my purposes). For the photographer, I'm sure it's a crapshoot.