Your correction is right, but I would add that "buy" is transitive and you don't normally use a transitive verb without at least implying an object. So the word that would normally be used is "to shop". At the same time, though, we tend to avoid saying the same word twice in a sentence (except when grammatically necessary), so "shop" (the noun) would likely be replaced with a synonym, like "store". I personally would use this version:
"Is this a store you always shop at?"
Just one more note of worth, though not terribly important: the original sentence is wrong for what they meant, but technically it is grammatically correct with a completely different meaning: it means to ask if you have a history of buying the shop. That is to say, paying to acquire ownership of the entire shop (and with it, ownership of the business that runs the shop).