Not a bug, but intentional. The Armoury has hugely better prices than the rest of the weapon traders, therefore it can only be accessed once to avoid making other traders useless.

Tip from my side.
Do the dungeon in thorwal first, so you got some more money to spend there to maximize the advantage.

Of course the equipment form the armoury would help, but you have to decide:
small advantage now or relatively BIG advantage after the first dungeon.