A lien can only be placed against a vehicle when work or services have been completed on a vehicle and the registered owner refuses to pay. That doesn't apply here - the dispute is over the sale of the vehicle.
This isn't a legal advice forum, so I don't want to get too far afield, but OP might also have a remedy in an action for the tort of conversion. But at its heart, this is really a simple breach of contract, which can be resolved with a small claims case if all other attempts to recover his property fail.
