A lot of locals sell their products on the beaches at a very extremly high price. They come and chat this and that with you and then all of the sudden you find yourself in a situation where these guys swarms you and shows all their prducts to you and trying to make you buy something. My mother bought seashells for 250? and we got the same amount and same looking seashells from another place(Pamplemousse gardens) for a 1/5 of the price.. The fruit is cheap and fairly fresh, and it's nice if your lazy. Be aware that the sellers tell eachother if you've bought anything from one of them, and they also keep track of the costumers of a hotel, so they know the potential costumers!
Try to negotiate about the price, u usually get a better price if you pay in euros, but make sure you get the right change, they are sneaky :)
Buy things at local markets, in superstores and shopping malls,...