"un gendarme" is a military policeman, who has the competences of a policeman, but working mainly by the countryside. "un policier" is a police officer working in the main towns. Under French law...
A small shop is a BOUTIQUE. The person who keeps it is a BOUTIQUIER. however, he is likely also (and first) to practise a particular trade, and to be a BOULANGER, BOUCHER, TAILLEUR, EPICIER etc.