Trading with pirates is, at best, risky business. If you havent already paid protection money, there is a good chance your ship will be attacked. If you sail directly to the pirates' warehouse, the chances are good that you will not be attacked. Make sure the holds of your ship are empty, otherwise they will regard your cargo as booty. Make sure also that you don't have any cannon on board. It is NOT possible to conclude a trade agreement with pirates, but you can still buy from them through their warehouse, even though it's risky business. Since pirates deal in stolen goods, and sail a farther distance than anyone else, they sometimes have goods to offer that cannot yet be found anywhere else.