Facebook marketplace scam

I sold an item on fb marketplace , bank transfer .

They inspected the item at my home; it was advertised in the photos .

They were happy with it and paid and left

An hour and a half later asking to return it for a refund.

Is this some kind of scam?

How do I protect myself here.

They’re saying the item is dirty and heavily used which it wasn’t , and they wouldn’t have paid and taken it surely if they thought that.

Thoughts.