Seems like every time I make a suggestion I end up making life harder on poor Zaira.
I like it, and think this is a good way to make cargo and crew selection more important and even get characters along for the voyage that would not usually be inclined towards it. I am unsure of how the numbers go (they may end up being too stacked in favor of the players) but my suggestion would be to handle it as a group modified roll. The crew (the player character members of the crew at least) meet with the buyer(s) and the negotiations start. For every piece of cargo that a character has training directly relating to (strict that it has to be exact to the nature of the skill), there is a plus 1 given on the 1d100 roll - with only the highest in that skill being counted and etiquette always counting as applicable.
So, Zaira, a bard, a weaponsmith, a jeweler, and a warrior are gathered around a noble's table preparing to dine and negotiate business. It is usually the captain's responsibility to make introductions and facilitate dealings - but Zaira knows she does not deal with people well (rank 2 etiquette) so she lets the bard take charge (rank 5 etiquette) who manages to captivate the noble and his underlings. Some chit-chat is made before moving on to the cargo - a shipment of weapons, jewelry and food. The warrior knows a bit of on smithing arms and armor (rank 1), and is able to contribute a little bit - but the MMR has sent along one of their Masters (rank 5) who (with some help from the warrior) manages to personally guarantee the quality of each and every piece of the cargo's weapons. Next up is the jeweler (rank 3), who while not working for the General Store can talk impartially about the high quality of the pieces he had no hand in making. Finally, the food is addressed - a nervous glance amongst the crew before Zaira firmly declares that the food is without a doubt edible (rank 0).
On their bargaining roll of 1d100, they receive a bonus of 13 (bard's 5, smith's 5, and jeweler's 3 with Zaira and the warrior offering no bonus).
If they fail the roll, the bonus if halved.
Option of each character only able to use one skillset each (preventing taking along one high level character who can stack up a 20 plus bonus on their own).