You can use the high-level multiplayer API of Godot, which is explained here: http://docs.godotengine.org/en/3.0/tutorials/networking/high_level_multiplayer.html
I would suggest you get familiar with this first.
This will require one of your players to host the trade room, and the other to use the IP address to connect. But you can also make a separate trade server and have both players enter the IP of that server.
If you don't want them to enter any IP address, then you will have to either hardcode it and make sure the server always run under the same IP, or have an intermediate server (using HTTP for example) which provides the list of available trade servers.
In the case of not entering any IP in a LAN context, you could use network discovery to search for trade servers in your local network (aka an instance of your game that opened a trade peer). I don't know how that works in detail, but it was mentionned here: https://github.com/godotengine/godot/issues/13947