Can you buy inside the building?

As others have said, after the entry fee is paid there is a cute little gift shop and trays of fish for purchase to feed the seals

Yes, there's a cute gift shop inside the entrance to the aquarium.

Yes

They have a gift shop with alot of cool stuff. There is an entrance fee, and then you can buy trays of food to feed the seals.

You can purchase the entrance fee, and some cut of fish to feed the seals inside the building. There is nothing else to buy.