I gave it a test this Franban.
Franban good was Saori and with R20
Crafts Trading +
Increase trade goods purchase i could buy 154 Saori.
With
Mer. Mentor's Wisdom 1 i could buy:
solo: still 154
2 or 3 member fleet: 175
4 or 5 member fleet: 195
But only my toon with crafts trading 20 could buy 195, a friends toon (in the same fleet) with crafts trading 10 could buy 175 and my 2 alts without crafts trading could still only buy 154.
Looks to me like it works this way:
1. It depends on how many people are in the fleet, but not liniar. (same amount with 2 or 3 and with 4 or 5 member)
2. It only works for toons that have the trading skill themself.
3. If they have the skill the final amount still depends on that toons trading skill, not on the highest skill in the fleet.