So let's start with the obvious, people in Yamatai can own Zesu. You can buy a Zesu knife if you are a member of the millitary for example. You can also get Zesu from ships given away in the salvage giveaway. In addition, you may be able to salvage ships yourself. Then you can use antimatter and subtractive manufacturing to make what you need. So obtaining Zesu in general doesn't seem to be legally a problem. What does seem to be protected is making Zesu, which is kept restricted to a very small number of companies. There might be restrictions on buying and selling Zesu except for a few companies in certain regulated ways.
In real life, the TLDR; seem to be that restricting ownership of a materiel doesn't seem to happen much and it's always poorly enforced. For example, if I wanted to have some Uranium, I could just buy it. Now, if I wanted to use it's radioactive properties, then there would be a lot of laws that apply, but if I just wanted to say make Uranium Glass (which doesn't use the radioactive property) or keep it as as a collector's item or forge it into a knife, I'd be fine. Another time materiel was restricted was Gold in the US from 1933 to 1974. However, it was still able to be owned in small amounts and used in things like art and wedding rings. Ultimately, very few people would be arrested for it. During the 60's The main problem with getting titanium was that it wasn't being sold to the US by Russia, which seems to mostly be the case.
I would suspect that Yamatai's law is that hulls and large pieces may not be directly sold, although they may be salvaged. It may not be used for ammunition or for any weapon beyond the mecha scale. It may be used as armor, but your going to have to source and cut that yourself, and good luck getting it in commercially viable amounts. There probably aren't any laws about import because until very recently, there wasn't anyone to import it from.
I imagine that Kuvexia's laws are more relaxed, but with how to make totally-not-Zesu a secret that is guarded very closely. You might be able to buy custom Zesu there, but they are currently in a lot of economic turmoil so IDK. The vast majority of the Zesu on the market outside of Yamatai is probably salvage and subtractively manufactured. It would be a very rare piece that is actually custom made.
How does Yamatai regulate restricted materials purchased in foreign markets?
It probably doesn't, it's a new problem. They probably regulate how it is used and what it can be built into. How to make it is extremely heavily regulated and protected. Their a lot less concerned about if you have it than if you can make it.
How open or available is that market? Outside of Soon's place?
Fairly closed and reliant primarily on salvage and subtractive manufacturing. Soon's place probably has some.
Where is the line drawn? Weapons? Materials?
I think it's probably drawn at weapons above the PA scale, and ammunition. Just having a piece you salvaged for armor probably isn't illegal.
Does SAINT care about the movement of restricted materials?
If it's in significant amounts, probably. But more in a information gathering sort of way to be on the lookout for potential threats. Information on how to make it going the wrong place is probably a quick way to get a special operations team dropped on your head.