The Guild of Merchants is a rarely mentioned London guild in the series.
It is likley that this guild was created at the time of London becming a traction city, however it is possible that they were an order at the time of Fever Crumb, although this is uncomfirmed.
They are largely unmentioned, although it is said that they are in charge of Airship landings and departures, as well as making city profits. They where assembled at the time of Valentine 's departure, and they are mentioned to have been wearing green robes. Their only other appearance was during Valentine's speech where the historians are said to ...not make as much money as the merchants... implying they are mainly or partially geared to making a profit.
It seems likely they were in charge of currency.