I was wondering, if you buy a mod, can you use it more than once? Or do you need to buy that mod for each store you build?

Usually mods are per domain installation

Thanks for that.

Welcome

It really depends on the licensing of each MOD. Check with the MOD author to clarify. Not forgetting that they are written by different people, so there's different ways of going about it.

Thats why i said "usually"