The thing is that arches themselves aren't commodities. People generally don't transfer them through physical relocation.great. let's both walk two miles, you and me.
you can carry thirty pounds of bricks, as well as mortar.
I'll carry a mental image of an archway & the knowledge of how to build it.
now, which of us will have an easier job transporting the arch? you or me.
Let's say Gilgamesh wants to build a stone arch, but doesn't have any stone. He goes over to visit Humbaba and says, "I want stone. If you give me stone, I'll give you barley."
Whereupon Humbaba says, "Sure." Note that at no point in this transaction has Humbaba actually had an arch. He has had stone, which Gilgamesh can turn into an arch, but that's not intrinsically an arch.
Now let's say that Humbaba wants to have an arch. So he goes to visit Gilgamesh and tells him so.* Gilgamesh says, "All right. Here's how you build an arch. I'm warning you, it will take a while before you're any good..."
Humbaba, thus, gains an arch, and with his stone he can make as many arches as he wants. But in order to obtain the arch, he needs to start with the conception that arches are useful. If you are someone in West Africa, in order to get gunpowder, you can either
1) realize gunpowder is useful before encountering it, or
2) randomly invent gunpowder, or
3) encounter people gaining benefit from having gunpowder.
1) is rather nonsensical. 2) is unlikely. 3) thus remains as a viable option, but it requires contact with whoever uses gunpowder, and if they're getting an advantage from it, they might be conquering you.
*Gilgamesh probably wouldn't give it up for nothing, but I wasn't creative enought to come up with an object of barter.