"Do they?" is correct. In fact, the correct pronoun for words like "somebody, anybody, nobody, etc" is "they".

"do they" is correct.

"do they?" is correct. further info: if the given premise is in the positive state, the tag question should be in negative state, and vice versa. "no one keeps they receipt, do they?" "everyone keeps their receipt, don't they?" :)

Julia, If you are trying to express the idea that no one (no individual person) keep his receipt, it is illogical to use plural pronouns in the rest of the sentence. "No one" is a singular pronoun and should be used with a singular verb in formal English. No one keeps his receipt, does he?.....is the correct form Some people are under the delusion that the use of the singular pronoun "he" is sexist, but in such contexts "he" is considered neuter both in English grammar and in law. Although the use of "they" as a substitute for the singular neuter pronoun "he" is widely accepted in Britain, Australia, and North America in spoken and informal English, it simply sounds adolescent when used that way in formal written English. As an alternative your sentence could be rewritten like this: People don't keep their receipts,do they? "People" is considered a plural noun.