Should I feel guilty?

[Blaaaaaaaah]

I know OP has already decided BUUUT Ima just say what I think anyway.

If I was you, I wouldn't feel guilty. That person has made a mistake, tough luck. Of course it'd be great if you're nice and give it back, and everything will be all good, but if you choose not to, I don't think there's anything wrong with that either. It's not like you scammed it or anything. He/she had priced it themselves, their responsibility to check, not yours.

I'm not even thinking about what's gonna happen if someone else came along and saw it. It's gonna be the same story.

Yeah, even if it was me who priced something wrong by accident, I'd be sooo depressed and hope that the buyer will come back to me, but if they don't, it's really my own fault and I can't call the buyer a bad guy because all they did was buy something from a shop at whatever price it was set at. Simple as that.