In practical terms, the individual person handling the transaction doesn't own the cheese that you want to buy, he is acting on behalf of the shop. Just as you would ask "do they sell cheese there?", English speakers who retain a "plural you" in their spoken dialect do ask "do y'all sell cheese?" or "do yiz sell cheese?" or even "do you guys sell cheese?". Unless you are going from person to person trying to find the one employee who holds the key to the cheese cabinet, you don't actually care whether the individual that you are talking to sells cheese, just that cheese is sold on the premises.
Of course, that makes the autonomous an ndíoltar cáis anseo? an option, but it does require a qualifier lo locate it in place or time, whereas it's implicit in "do you sell cheese (t/here)?"
When you turn a statement into a question by putting the interrogative particle an in front of it, you eclipse the verb (if the verb starts with a letter that can be eclipsed).
Béiceann do mháthair - "Your mother yells"
An mbéiceann do mháthair? - "Does your mother yell?"
Canann tú - "You sing"
An gcannan tú? - "Do you sing?"
Díolann sibh cáis - "You sell cheese"
An ndíolann sibh cáis? - "Do you sell cheese?"
Actually, Irish might be a little bit more straightforward than English in this particular case. Which is the verb in the question "Do you sell cheese?" - "do" or "sell"? If "do" isn't a verb in this case, and just serves as an interrogative participle to turn the sentence "you sell cheese" into a question, then it is exactly equivalent to an, which just sits in front of the simple statement díolann sibh cáis and turns it into a question.
It's in the response to the question that English and Irish diverge, because English uses the same response for all verbs ("we do", whether the question was about selling or reading or eating, etc), whereas Irish responds with the same verb that the question was about, díolaimid if the question was about selling, itheann sé if the question was an itheann sé cáis, canaim if the question was an gcannan tú.
It would be unfair to mark it as correct, because that might leave you with the mistaken impression that An ndíolann tú cáis? means "Are you selling cheese?".
The hint probably came from an exercise that started with an bhfuil tú because English is a bit inconsistent about how it turns a Sentence into a question.