Is there a way to improve something that other users can't use when someone is already trading on a multi server?
I looked up the wiki, but I couldn't find a way

This is actually a limitation of the game's default Merchant system.
The game doesn't allow two players to trade with the same Merchant at the same time, so this isn't something caused by the mod itself.

Fixing this would require creating a custom trading system, which is planned for other reasons too, but it won't be happening anytime soon.