I keep picking the stuff up but it wont go above 300. I know you use it to trade to the Nightborn but I can’t do that yet. Seems kinda low. Am I missing something?
There are several treasures hidden around Suramar that increase your capacity. There are also a few rewards in the main Suramar quest chain. In total I think it can be as much as 2000.
[edit] The comments on this Wowhead thread for the achieve for upping it to 2000 will probably help: