The inventory of the Masters in your hideout can include some pretty nice purchases, but resetting the inventory is done differently than most other vendors in the game. In order to reset their inventory you need to accept a new mission from them. This is the only trigger to actually reset it, and provide you with a fresh inventory. You can either complete their daily mission, fail it, or simply abandon it, and the inventory will have refreshed. To abandon the mission, type in "/abandon_daily" without the brackets, while being inside your hideout.