D&D books, 40k minis, star wars collectibles, etc, I think one would be aiming to be a SFF store as opposed to a SFF bookstore, comic books and such maybe. There are stores that aim that way, your average gaming store just more books, those usually specifically carry SFF bestsellers and game-based novels. I've never heard of one turning much profit though, most I know of dip under quickly and the ones that don't tend to perennially squeak by, but I haven't looked much recently, geekdom is a bigger niche then it used to be, and no longer limited to teenagers and poor college students.