I have received a lot of questions about used car lots in Panama. While I was waiting at the hospital today, I found these sites listed in the want ad section of La Prensa. These sites may give you an idea of what is available.
http://www.fullcarspanama.com/
I often recommend people talk to the dealers of the brand car you want and ask if they have any clients that are wanting to trade cars. You will also need a mechanic you trust to inspect the car and advise you of its real condition.
