What can I do if an auto dealer has not paid off my trade in?

You have covered the options quite well in your question. Usually, the cost effective route is to work through the agencies you mentioned, although it is hard to say what kind of results you will get and when. Yes, you can hire an attorney to sue, but the question is how far it will have to go. The costs will mount quickly. So, if you are not prepared for that, then you need to take your chances with the route you are going. Even if there is any chance of recouping attorney's fees, it is not likely that very many attorneys will consider a contingent fee. There are firms to which we refer consumer matters. A firm with that concentration may be more inclined to get involved. A BBB complaint may be helpful if the business responds. To protect others, post your own complaint on the BBB website and any other sites from which the dealer may be getting customers, but keep your complaint simple and to the point of what happened. Finally, if the dealer is a franchise of a larger company, you may try to move up the ladder with a complaint.