If you find something you are interested in, get the tag numbers and run them on this site. It is the official registry for Costa Rica and should give you all of the numbers you would need to verify.
http://196.40.22.13/
Run the plates on this site and it will give you the history of traffic infractions either paid or outstanding. If it comes back with an outstanding ticket, I would look for a different vehicle as it could be more complicated than just paying the ticket.
http://www.csv.go.cr/cosevi.html
Run the plates on this site to see if the annual taxes are current. If they arent, someone will have to pay them before the title can be transferred. A 45 pickup would be "carga liviana", a 40 is "particular".
http://portal.ins-cr.com/csoa/csoaportal.aspx