Not sure how it is right now but they have an interesting spin on it.
Basically you will be able to choose between being able to trade on the market OR better drops.
So if you want to play solo self found, you will be rewarded.
You will be able to switch between the two but you will have to level it up again from 1.
Source:
https://forum.lastepoch.com/t/trade-development-update-introducing-merchants-guild-and-circle-of-fortune-factions/51994They will also have items that drop if you are playing with a buddy that you can then use on an item to give to them.
Leveling in merchants guild will allow you to trade and get discounts on trading. Also unlock more slots in marketplace.
Leveling in circle of fortune will drop more unique's with legendary potential, more exalts, easier crafting.