I am an electrical contractor. We had a list of work to be done for a home that was being sold. The buyers' agent sent us the list and cc'd the seller along with a copy of the agreement between the buyer and the seller that the seller would pay for the repairs. We got a downpayment of $500.00 from the seller, but she hasn't paid us the bill's balance (about 3K). It has been 4 months and the seller is not answering our calls. What is the best next step?