First you need to reclaim a city using your Assassins.

Then you must go to Piri Reis, speak to the shop keeper and buy all of the items. Then sell those items straight back to the shop keeper.

If you want to be able to get more items you have to do the Piri Reis missions with your Assassins or near the shop keeper, towards the right, there are four pin up boards.

Interact with them (B) and do the missions for the pin up boards.