To buy something means to go to the store and purchase it.
But there's also an idiom, "to buy something," which means to accept, or believe something which a person tells you. (This is the meaning of your example: "Hey! Did you actually think that what I said was true?"
"Did you actually buy what I said?")
Your version would be okay with a capital and a comma: "Hey, you . . . "
I thought you were going to be careful with punctuation and capitalization.
French
quickly , please .