So this is an interesting thought but the way the game is designed it is sort of already in the game right? You could try and communicate with another crew and trade goods with one another by withdrawing and depositing resources on each others ships just like any pirate crew would in "real-life". The game relies on the social aspect of fun and the freedom of allowing the players to make their own decision by communcating with each other. By introducing a pure trading system the players no longer rely on that channel of communication between crews but the system instead.
I think it is those unwritten verbal "contracts" that make the game much more interesting and pirate like. Do you trust the other crew to keep their end of the deal? Do they backstab you after getting what they want?