He can file the lien for storage. It is an easy thing to file a mechanics lien - as long as the form is properly filled out it can be recorded. You may have defrenses to it and you can make a motion to vacate the lien. As to selling your car he would have to go to court and foreclose on the lien and get a judgment before he could sell the car. I suggest you consult with an attorney and develop a strategy to vacate the lien and get your car back.
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Can he legally put a mechanics lien on my car for work not performed and storage of the car? Storage yes. Work not performed no.
Also, can he sell my car before a judgement on the mechanics lien is made? no, and even once the lien is put on he cannot auction it. The lien must be foreclosed on.
You hsould consult with an attorney to advise you.