This would help the server's economy allowing player to more easely interact. You'd type a command and an "inventory" would open, there you would see everything other players are "trading" for, it could be services or ressources, even ships. You would place an ad , choose an item that would represent your trade, a title and a description and then publish it. An annoucement in the chat every hour would tell people that a new command is available. I know a similar plugin already exist on skyblock server, pvp etc.