[Bishop Edward]

Well, Trade (both import and export) in Thay is controlled by the Guild of Foreign Trade which is headquartered Bezantur...

but since we're talking about completely internal trading (mostly to and from adventurers)... legal goods would be controlled by the Tharchions' offices, and of course certain groups have a monopoly over illegal trade...

so I can't imagine that 'realistically' an independant merchants guild would ferret well unless of course their profits were either being spirited away to the Tharchion (which is the case in the Guild of Foriegn Trade), or acting as fronts for the illegal traders (which already exist to some extent), however, both of those possibilities would preclude players making any more coin... since the bribes would be just as high... and the cost of operating the merchant just as high...

You may be able to eliminate the merchant's fees though ... by having a slave act as a shopkeep...