To me the randomness of what sells well seems all about whether your image ends up high on a popular search. I have a couple of images that sell every day and multiple times most days. There is nothing special about the images but they do come up high on the first page of a most relevant search that produces 2 million results and have been there for a few months. Even if the image was better than 99% of the other images in the search that would only put it at page 200 of the 20,000 pages. And I would expect that it would never be found even on page 200! So if you expect to compete in the very large popular searches it will be just random luck to be able make the first couple of pages and have the image found. The better bet is to produce good images in more specialized areas where they can at least be found. That's not going to make you rich though so if you're in this to make any more money than will buy you some good equipment, you will probably be disappointed!