I think there are 2 factors which affect my success in finding books in charity shops. what books are sold: I would like to understand the proportion of books

I think there are 2 factors which affect my success in finding books in charity shops. what books are sold: I would like to understand the proportion of books sold by genre. Shops have a very high proportion of thrillers and crime novels. likelihood of donation: Perhaps some fans are more likely to keep books than others. In my donations, thriller & crime books are donated first, I keep my SF. This leads to a low chance of success in finding science fiction paperback for me. I guess SF (myself included) are just hoarders.....