Frontier airline tickets

I have a credit with frontier airlines for $517.20 and i am trying to sell because i can't get a refundi have a total of $517.20 towards tickets for frontier airlines someone used my card withoit my approval and didn't get on the plane and there is no insurance on them but they would give me credit. Can someone please help me out by buying the tickets it can be used for one person or towards the whole family please help me out thanks