I had the idea that there are several different witch covens in the local area, like there are many vampire families and presumably were packs. In order to own a business like Aggie's you would need to be aligned with a coven and can only sell to members of the same coven (and maybe a few allies).
Aggie isn't aligned with a coven and sells to anyone, which is why her business isn't strictly legal. I got the impression from the last post that Cam is kinda blackmailing her to help him out in return for his silence, which I like. Perhaps Aggie's lack of alignment is a reoccurring conflict.
Also maybe vampire families employ a witch, since I have ideas for Aggie's and Ian's history. What kind of vamp lore do you want to use, by the way?