What should I do if I ask for a refund again if I sell my cell phone secondhand through a personal transaction?

I sold my cell phone second-hand through a personal transaction.

They want to get a refund again because the buyer changed his mind.

What should I do if I make this request?

Do I need a refund?

Or can I ignore it because it's a simple change of mind?

1109 2017-04-26 Gentleman

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