I don't know why no one has mentioned the traditional trades of being a merchant of food stuffs and other products.
I'm really big on nutrition and healthy eating. There is a large movement of organic, locavore, fair-trade no-carbon high-end food stuff with relatively big margins by comparison to conventional stuff.
Planning on following the harvest season up and down the america's, can probably buy bulk, and then dehydrate or otherwise preserve foods, (i.e. on a plastic sheet on a secluded beach somewhere, and vacuum sealed for transport) and then sell them to both cruisers and health stores.
Have a nice little sailboat on the package "no-carbon sail transported"(intend to go engineless), finally those locavores can have their mango's, banana's and coconuts without feeling guilty about the footprint of trucks, airplanes and freighters.
There are already some companies shipping wine by sail in europe BBC News - Sailing into the future of global trade?
and there are some other examples of more local stuff with yachts.
Ya food might not have the best margins, but there is also gems and semi-precious stones, which are cheap in south america and caribbean, but should sell for a pretty penny up in north america. The guy with the best price-point I met a gem show, said he mined it himself in brazil, to cut out the middle-man, and he had to fly there and back, so limited baggage and lots of oversight. Probably would need a prospectors license or something in country for anything more than souvenir amounts though could be worth-while.
Again the food is a good cover since you don't really want to be known for hauling gems around, makes pirates drool too much, so would have to be a small amount used for ballast. Another benefit of the gems is can use it for the minor bribes which are common in the warmer areas of the world (they could give it to their lady friend if they don't like it themselves). Other than gem-shows there are also new-age shops which pay big dollars for gems, or at least sell them at very high prices, who knows maybe some of their customers would also like low-footprint fair-trade gems.
Of course one of the best things about the transported preserved food, is having high quality provisions on board all the time. Assuming business is good can scale up and get or build bigger sailing boats, and have more crew. Could also make a liveaboards co-op kinda thing, where people can join follow the procedures protocols to obtain and process the foods, and then sell them to affiliated retailers.