Use 'belongs' instead.
The dialogues' grammar is fine, but you should make everything else not-dialogue in past tense.
EDIT: Btw the one you're asking about? We are following a tone there, "Hearts are not to be bought but stolen, they say" is the more proper one. The one I posted earlier follows more precisely to the actual wording (you would kinda pause a bit at 'bought') though it looks weird in writing. Or just two ways of speech.
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