Except apparently he was scammed. From the way they said it, it sounds like the guild has fixed rates, and likely the clerk only paid that amount pocketing the rest.He paid a gold coin. He can set up his stall wherever he wants.
That said, it sounds like it is a huge problem if the mercantile guild has their clerks scamming their customers. That is kinda the guild that depends more on trust than any other guild. No wonder the market plaza was entirely void of peddlers and merchants etc.