Limits to buying from NPC / AI at trading post?

[Crai-Crai]

Sometimes I'm a bit short on production of certain goods and don't want to add a whole chain to cover it (looking at you, canned goods!). So I set up buy orders at the trading post, especially if there's no NPC that sells it (like steel beams at Archie's).

Sometimes the NPC (or AI that I have trade agreement with) will show up and top up my inventory to the amount set. Sometimes I set a buy order and no one shows up to sell it.

Will some NPCs only sell certain goods? Are there unlock conditions for certain goods (like if you don't have the production unlocked yet)? It's not obvious and I couldn't find anything helpful in the wiki.

[Tankfriend]

NPCs have two options for selling you stuff:
a) They sell you whatever they're also selling on their own islands. This is a limited quantity of goods, but the quantity will "regenerate" over time.
b) They sell you whatever they have bought on another island or whatever was sold on their own island This is limited to the exact amount of goods they have acquired.

For example, if you have no NPC trader that sells steel on their islands, and no player in the game sells steel to the NPCs, then the NPCs will also never sell steel to you.
If you then go and sell 100 steel to an NPC, the same NPC will later sell you up to 100 steel if you set up such a trade order on one of your ports.

[Crai-Crai]

That makes sense, thanks!