"She sells maps of the country."
Translation:Hun selger kart over landet.
It's a set phrase, maps are over areas.
If you need a map set to use with your gps/boat/going biking/insert area of interest or technical term here, it's either "kart for båt" or "kart til båt" (maps for use with boat) "kart for skituren" or "kart til skituren" maps for skiing (ie maps where the tracks have been replaced by trails for skiing).
because that would be like saying 'a map that belongs to the country,' just like you could say 'barnets lesebriller' (the child's reading glasses); when really you're trying to say 'a map that shows/illustrates the country'