The good news is that it is easy to sell, the bad news is that you have to find a particular kind of buyer - one who is willing to "import" the car - process all of the paperwork and pay the taxes. Hopefully you have all of the documents they will need to nationalize the car (Costa Rican vehicle use permit, original title in your name, emissions certificate, etc.)
(for anyone looking for information on buying cars here, look at this article:
Buying a car in Costa Rica - top tips )
The important question is how long the car has been here, you only have 3 months plus 1 three month renewal, if you leave the country and come back.
The easiest option would be to talk with some used car dealers, maybe exchange your car for one that you want or allow them to sell it on a consignment basis and they can take care of all the administrative paperwork.
The buyer will have to nationalize the car. So they will have to pay all duties including the value customs assigns for "freight" ( 7% ) and "insurance" ( 1.5% ).
If you want to sell it yourself you could try an ad in La Nacion, the Tico Times and AM Costa Rica. You only have to specify in the ad that the car has foreign plates, or tell the people when they call.
