shopping on Sunday in St. Martin

Are stores open or closed on Sundays ?

You mentioned St. Martin so I will only give you my thoughts on St. Martin rather than St. Maarten.

Most of the Marigot shops are closed on Sunday, including the open air market. Restaurants will be open.

Last week, I asked at Little Switzerland and Shipwreck Shop, as well as our friends at Trident/Shiva's. Consensus is that from November on almost all shops will be open, even if only one ship is in on Sunday.