What Matrix said for provisioning at Marigot Bay. There is a small market at the Moorings base, but you would not want to provision there... too small and not the selection you would most likely require.
I found fresh fish is more readily available down south versus BVIs... if you enjoy fish, plan that into your menu. It is very easily purchased through boat boys who will clean and fillet it for you.
In addition, I found restocking a little more difficult down south. Keep in mind stores on the smaller islands close mid day usually until 3 or 4 pm. Depending on your itinerary, you will most likely eat on the boat more than in the BVIs, so proper planning is necessary.
We stayed at both True Blue Bay and Le Phare Bleu. True Blue is closer to the airport. I wouldnt hesitate to stay at either location again. Both very charming properties.
Down Island sailing is an awesome adventure!! Enjoy!