I think a few things need to be clarified here. Central America is not easy to buy, sell or import a vehicle from North America. I have been going to the region since 1998.
First, getting a bike imported into any country is not easy nor cheap. if it was, there would be a lot more bikes in the region, which there are not, but more than 5 years ago. Keep in mind borders are a pain, and you should get there in the morning, allow 2-6 hours for each country border Customs/Immigration, as you need to leave a country properly with paperwork, and then enter the next country, carry plenty copies of Passport, Title, Regestration for the official paperwork. You are basically temporarily importing the vehicle when you enter, then exporting it, then cross border, and import it, again. You also need to secure the bike at night, in a home, hotel or fenced in guarded area, never leave on the street/open areas. Read the Driving the Americas forum, as many people due this with cars/RV/Campers, and the border issues and routes to avoid are all in the details.
If you are just touring the region with a bike with North America plates, you of course need the original title, in your name, same as Passport. Each country you enter, the bike goes in your Passport too, as a Visa, for the same time frame as your tourist Visa. Depending on country, but 90 days for the C-4 Guatemala, El Salvador, Honduras and Nicaragua. 90 days Costa Rica and 180 days Panama. Belize is 30 days.
If you exit the country without the vehicle, you will have to pay the import duties/taxes on the vehicle, and they determine the value, not you, and its not in your favor. Import taxes range from 50-100% depending on country.
Costa Rica is the exception, the vehicle does not go in your passport, so technically, you could leave it there, or, at the border.
But in order to sell the vehicle, with North American plates,you need to go thru the legal process, usually best with a broker and lawyer,to import the vehicle, and get Centro America plates on it, and this can be a long process and not cheap. Most buyers are not interested as the tariff/taxes are usually as much as the vehicles net worth, so its not as easy as 1-2-3.
For someone to buy a North American plated vehicle and register it properly, the paperwork and taxes/duties import fees, is usually a great value that the bike itself in some countries.
Also, keep in mind the various safety issues associated with each countries rider/operator laws, like Costa Rica, you must wear a Bright Reflective vest, and a helmet. Anyone that has ever visited Central America, would no dare ride a motorcycle without a helmet, and also safety jacket, pants and boots. The leading cause of death in traffic accidents, is motorcycles, over 28% in Costa Rica alone. Its all two lane roads, no shoulders, pot holes, people, animals, farm equipment share the road, and the Pan Am is cargo truck drivers that tailgate, pass on turns, and its mostly mountains, and rains from May to Dec in higher elevations after lunch.
There is also a huge boom in motorbike imports in Costa Rica, mainly Chinese and Japanese, but even KTM and BMW are now common, and not cheap. The BMW F800 and the KTM 450 EXC seem to be the favorite of expats and locals. Triumph 800's are also more common, but I dont think there is a dealer in SJ, not sure. Suzuki is big player, and Yamaha, and every two bit homegoods store sells the Chinese crap, like hot cakes I might add.
If you want to know some great routes in CR to motorcycle on great roads with little traffic, let me know, I just spent a month there, my 40th trip.