As long as you've paid off the car loan in full then you should have no problem selling the car while someone else is listed on the car loan as a cosigner.
A cosigner signs a car loan along with you to guarantee that they will be liable to pay the loan back should you not be able too.
Either way you won't be able to sell the car if you still owe money to the car loan company for the car.
You'll need to pay the car loan off and then get the lien removed from the car before you can sell the car.
But having a cosigner on the loan should not affect your ability to sell the car as long as you've paid the car off.