I picked Unique and W231... you can load just about anything with those 2. You could probably substitute Bullseye for W231, however. You can't, however, ignore the slow powders like H110/W296, and 2400. In rifle cartridges you can get away with 2 powders... but not all pistol cartridges are the same way, generically speaking you would need a fast (W231,) medium (Unique,) and slow (W296) powder on the shelf.
Having 3 powders that are minimally sufficient to load most handgun cartridges is not a suitable approach for a store. That's like having a grocery store that stocks one kind of meat, one kind of green vegetable, and one starch because, between the 3, you can make an adequate meal. Nobody is going to shop at that store. It's not a business plan.