My Ex Boyfriend and I were together for 3 years. I took out a loan for him to use in his custody battle for his son, I also put a boat in my name for both of us. He has verbally agreed to pay me back until recently. I have his furniture, TV, and other belongings in my house. I do not want to give them back until he pays me back for what he owes me. Can I legally withhold these things (they have been in my house for 8 months since I made him move out, and I own my home) until I receive payment? Can I sell these things to get some of the money he owes me?

No, these are two separate issues. You have to give him notice to pick up his property according to your states requirements to allow him to pick them up before being disposed of.

If he owes you money, you can sue him for that, as he could sue you for the value of his property if you don't return it.
Also you have a boat in your name, he has to return it to you, since you own it.

I wouldn't advise taking matters in to your own hands. I would suggest if you have a written agreement that your ex will be paying this money back to you that you go about it the proper way and go the small claims route. Send something in writing give them proper notice and proper time to respond and if that doesn't work. File a claim. If you are unwiling to file a claim let them know that they are always free to come and get their belongings, you don't need them to bring the law your direction. Be fair. Good Luck