I'm thinking about keeping my ex's Xbox.
Why would you keep his Xbox?
I'm either going to sell it or I'm going to force him to pay me $50 to get it back.
I get that you want to get back at him for hurting you but you shouldn't be holding his expensive stuff for ransom. It isn't your so you should just give it back and move on with your life. Being petty like that will only cause you more problems.
I'm going to sell it to make sure that he doesn't just get it back.
Selling his Xbox is a bad idea, because it's not yours and if you're caught selling something that doesn't belong to you then you can get in legal trouble. What you're describing sounds like theft, so I think you should reconsider to avoid getting in trouble.