Since the prior owner operated as a sole prop., you could go after him personally.
Unless he operated out of his house (or still owns the building the new buyer operates in), a mechanics lien may not attach to the building you performed your work. The owner of the building did not contract with you.
The fact that the PO ran up bills/debts is of no concern for the buyer. The buyer is under no obligation to pay the PO's bills. (Truth be told, if I were the buyer, I would tell you go back to the PO.)
The buyer's verbal promise to pay the debt is as good as the paper it is printed on.
As Eagle said, your best bet is to take him to small claims court. Be prepared with your documentation of the work you performed, your cost of materials, and your time. Have your girlfriend available to support you, if necessary.
As many people before you have learned (I am on the list), use it as a learning experience. Next time, write up a contract/agreement of what you will do. Have him sign it. Request a deposit before you start work. Some states limit deposits to a % of the contract (33%). Check local law before you write. Demand payment upon completion.
If I collected on all the work I've done the past 20+ years that didn't pay, I could purchase a new JLU Ruby!