A week later, he wanted his BAR back, said I had taken advantage of him.
I know the feeling, I bought an AR-15 from my FiL about 3 weeks ago. He hasn't had a kind word to say to me since
I don't think he regrets selling it, I do
think he regrets selling it for the price he did.
In your case, at least it's a stranger. I'd do as Aguila suggested - ask straight out what he wants to do.