The problem with merchants is not the prison itself, it just even stronger there.
The whole island is isolated and far away from the typical trading routes. There is a reason why the Seekers are so desperately trying to get away from the island. The can't leave but get tons of fancy stuff? Also they have all those fancy skill books, but noone of them can learn any kind of healing skill?
Also as somebody already mentioned: all those fancy treasure stuffs like Bracchus get easily old, because you can buy better stuff soon.
If you tune down the enemies it would not even hurt to have less fancy stuff to buy. But on the other hand tuning down the magisters would break the immersion from a different point, I guess. You kind of can kill to many Magister who seem to be to weak for the real felt importance.
