possible asynchronous trade abuse scenario

the only current way of expelling other player from hideout is by using NPC alva in poe2 and helena in poe1 , to change hideout template.
as far as i know, there is no cooldown to change hideouts, manually doing so you can change hideout in less that 5 seconds, so i would assume that with bots this process could be faster and on repeat.
this would prevent trades from being realized, creating price fixing accounts

Good point. I already experience issues joining someone's instance/server/hideout sometimes and they have to come to me instead or vice-versa. I don't think async will make that problem lesser. I think GGG will put in efforts to curb degen behaviors. But, unfortunately, it will be mostly reactive and based on loudest complaints (though I do believe they have some botting detection tools they use as well).

From my understanding of how instances and merchants function it should work regardless, whether they are in the same instance or not if that merchant is/ was theirs it will contain their items and you will be able to purchase them.

I do not think it is possible to switch hideouts fast enough to permanently return an error, I'm not certain but pretty sure just the loading time alone would prevent this abuse case. I also believe if you are in the state of switching it won't return an error for the buyer, but rather just teleport them to the last loaded hideout (which would include the merchant and items).

Basically, there can be as many copies of "their" merchant as there are instances of their hideout. The merchant does not "follow" them to their instance. Another instance with another merchant is just created if the seller switches instances and leaves you in an old one, likewise if you somehow load into an old instance the hideout and merchant should still function exactly the same.

Price fixers somehow bugging out the site/game to make the functions return an error would be devastating for the economy though, really hope there's none of that