1. Something actually he sells, at least brand/maker wise, IMO if your gonna sell it carry it.
2. I like to see that they carry a variety of price points and styles. Helps make me feel that their selection in shop is solid and that the owner trusts in his stock.
3. this only applies once IMO, so if you want to carry an amazing custom piece, or something from a maker you don't (yet) carry as a please do as we all like seeing such things, but carry a secondary blade as well.
So I guess at the end of the day, I don't really care what they carry, but that what they do carry shows trust in their product. More simply put don't stock things you wouldn't be willing to use yourself.