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You apparently "sold" the vehicle to someone else without transferring title or extinguishing th existing loan. If so, then your lender had a priority lien which cannot be extingushed by your "selling" the car to someone new. That person took possession subject to your continuing debt to the original lender. If you didn't pay the loan off, then the lender could repo the car, because of its priority lien.
The person to whom you sold could complain that you tricked them into believing that the vehicle was free and clear, when in fact you still owed a debt to the lender. If they succeded, then your sale would be rescinded and the buyer would be entitled to their money back.
You could argue that you should be entitled to the reasonable value of the use of the vehicle for the time that the buyer used the car. That's a reasonable argument and a judge might agree, unless you were found to have defrauded the buyer, in which case, the court would probably refuse to allow you to keep the money, because the buyer was relying on your statements about the car when entering into the transaction.
Hope this helps.