If you want ammo to sell in your own store, this is what you have to do:
1. Go to "Ammunition Artisans" and the press "Wholesaler". In there you'll see a lot of different ammo to buy.
2. Buy the ammo that you want then you...
3. ...go to "Fronts" and press your ammo store.
4. Once inside you'll see at the buttom all the ammo tah you bought at the "wholesaler". Add this ammo by pressing "Add"
5. Now you have filled up the ammo stock in your ammo store.
Cheers, mate!