How to sell in a shop on a minecraft server?

So I'm playing on a server, and when you first get into the server you go through either of the 3 portals I went through the skyblocks one and there is a lobby and a shop. I started playing my skyblock by typing /is and now I want to know how to get back to the lobby with the shop so I can sell some stuff? I can't seem to figure out how to travel there with my stuff to sell.

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