Where exactly are you going to "purchase" those items? The other guys fortress that has been raided by the zombies?
Realistically, we will need to survive for a year or so after a total collapse. By then, many will have starved off or died of disease and it will be safer to garden out in the open and move about but there still won't be places to purchase anything. I would much rather trade heirloom seeds and ammo over a hunk of metal.
Likely wont be coming to your fortress, likely won’t be coming anywhere that isn’t on a shoreline in a warm place,
maybe not even in this country
LOL at concept there won’t be anyone anywhere to trade shiny metal world-recognized denomination currency for other things
people looking to trade ammo will looking for traders with a certain type of weapon, a rather limited market, and how far from their fortresses and by what mode of transport are they going to travel to barter ammo?
the people looking to trade for seeds are those not hungry now, those who have 3 months with time and place, weather conditions and safety to plant a garden, water it, and defend it. Again, a rather limited market....your needs and those of your trade partners will be rather small isolated groups, which is probably fine for your situation, if you have thought it through and things go as you expect weather and safety wise and there is no need to change your plans, I always expect the worst, which is the unexpected