The monopoly nature of these guilds and counting houses actually came about naturally, as only by sticking together, collaborating, and fixing prices could these businesses stay alive.
Otherwise, these gold laden geese would have been long slaughtered by the greedy nobles and kings.
Hence in Tibias, it had by now even become almost a custom for sons and daughters of the large trading houses to marry each other, turning all of them into nearly a giant family run conglomerate.
This was also mainly how these guilds and counting houses found the guts to swap client accounts like that- knowing the other side would have a hard time swindling them when both of their grandkids called each other grandpa.
Now, this was the advantage consumers got from these trading houses being a monopoly.
As for the disadvantages, well there is little point in beating a dead horse. Everyone knows these.