The amount the lender makes from the sale must go to cover the balance of the loan and costs of the sale and repossession. The lender must return any money left over (any surplus) to you. This does not happen often.
If the amount the car sold for does not cover the loan and expenses (if there is a deficiency), the lender can sue you for the full amount owed, including repossession fees, auction costs, and legal fees. Example: You owe $10,000 on the car. Your lender sells it for $7,500. The deficiency is $2,500 plus any other fees you owe under the contract.