I guess that 'A régi áruháznál teniszezel?' needs to be accepted, too.
yes, it's a correct translation, too
Do you can also be singular, however it's not accepted.
I think this sentence should actually be translated as "A régi áruházban teniszeztek", which doesn't actually make that much sense. "By" or "next to" would be more appropriate than "at", here.