In theory, the more people that do the Harvest missions for your Society the more items that become available from that merchant and (rumor) the lower the prices.
Note that every time you change Society any work you have done is lost. And currently most users change Society a lot in order to get access to the starting one (books and supplies) and their advanced crafts. Every time this happens, any quests done to harvest are lost.
I am not sure how they will resolve this, as there is currently a bug were users will often lose their Society status after logging out or playing another character (reported).
BTW Your society may be the area that generates your trade instance. For example The Charter of Truthbringer in Castleview.