Asked by user in California.
This guy gave me an "Xbox acc" and I put it up for sale, and now he's saying that he's going to sue me because he wants it back.
In California, a gift becomes the legal property of the person who receives the gift. At that point, the person who receives the gift is free to do with the gift whatever they please.
Consequently, the person who gave you the Xbox can't sue you for deciding to sell the Xbox. The only way this person would have a viable claim is if they did not intend to give you the Xbox as a gift but instead, for example, only intended to loan you the Xbox.