Lien sale
The process by which a storage lot sells an unclaimed vehicle to recover unpaid charges, on a timeline set by state law.
In practice
A lien sale is the end of a process, not a sudden event. Unpaid towing and storage give the operator a claim against the vehicle; after a period fixed by state statute, and after notice to the registered owner and any recorded lienholder, the vehicle can be sold to satisfy that claim.
Two things surprise owners. First, proceeds go to the charges before anything reaches you, so a long storage period can consume the entire value of a sound car. Second, a shortfall does not necessarily end there — in many states the deficiency can still be pursued against the registered owner. Defective notice is the most common ground for challenging a sale, which is why the address on your registration matters far more than it appears to: statutory notice goes to the recorded address, and a notice you never received can still be legally sufficient.