Selling seashells by the seashore is actually not such a great idea. Why? because 1) often the shells being sold were taken with live animals inside, 2) empty shells are important future homes for animals like hermit crabs; 3) some shells are from endangered species, 4) the usual “if you take it, it wont be there for the next person to see…,” 5) in some places it is illegal, 6) the remains of shells and corals and other organisms disintegrate to form the fine sand that forms beaches, and 7) selling them encourages the idea of touching and taking from the environment.
An alternative is making crafts from trash – bags from plastic labels, purses out of can flippy things – basically, reusing whatever is available. The materials are free, instead of harming the environment you are helping it, and – tourists will be happy to support a sustainable practice over a damaging one, and probably pay more for the item.
Programs to teach craft-making with trash are already being conducted, such as by the group Amigos de Isla Contoy.