You are liable for negligence for the property damage to the parked car you hit. The owner received $3000 less for the trade-in than he or she would have received had the car not been damaged. The lost amount on the trade-in would be the owner's damages (monetary compensation the owner could seek in a lawsuit for negligence against you).
If you have auto insurance, refer the matter to your auto insurance company which will handle it for you.
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