Yes you owe the money. When you sign for a car loan you owe the full amount. Since they sold the auto for less then you owe you are responsible to the lender for the balance. I am sure they are charging you for the repo fee, legal fees etc. They tack all that onto the loan balance including interest on the loan.
You can contact the lender and try to work out a payment plan or they can take you to court for the balance and probably get a judgment against you.
I know that this NOT what you wanted to hear, but this how it works, sorry