Trading Ground? IDK, it will be basically a synonym of each word, just depends on what you want to use. Better to know when the setting takes place to get a better idea of how to word it.
Usually 'area'/'region' or more informally 'patch' I'd say. You could even stretch it to 'turf'/'manor' if the sales are of a dodgy nature.
Agree with @SethLoki that area/region would work. I have also heard of companies that refer to sales territories as "districts".
