Selling a Promissary Note?

My ex boyfriend and I bought a house together a few years ago. Last year I sold my portion of the house to him for a lump sum and a promissary note for about $20,000 that was written by my attorney. After six months, interest started to accrue at 5% APR monthly. The due date on the note was last month, and he has not paid it. I have since moved 1,000 miles away and do not want to deal with the hassel going after the money. Can I sell the note? How much of a loss would I be looking at? Where would I go to sell? Thank you for any help.