There are a few different ways to go about selling a car with a co borrower. The first option is to simply sell the car to another party and have the new owner take over the payments. This can be done by finding a buyer through classified ads, online listings, or word-of-mouth. Once you find a interested buyer, you'll need to have your lender release the lien on the vehicle so that the new owner can get their own financing and registration in place.
The second option is to refinance the loan into your name only and then sell the car. This process will require working with your lender to get new loan terms set up, but once that's complete you'll be able to sell the car as normal. You may want to consider this route if you can secure a lower interest rate or monthly payment, which could make selling the car easier.
No matter which route you choose, it's important to keep communication open with your co borrower throughout the process. They'll need to be aware of any changes with the loan or sale of the vehicle, and should sign off on any paperwork before anything is finalized. Selling a car with a co borrower doesn't have to be complicated - just be sure to handle everything in a transparent and efficient manner.