There should be a /auctions or something like that, you hold an item, and do something like /auction (amount) and people can put in amounts, and the highest bidder wins, like a real auction!
You could also just run it like this so it doesnt spam chat. /auction start --An auction has started type /auction bid to bid-- /auction bid 10000 --Your bid is the highest bid-- /auction bid 9000 --The Highest big is 10,000!-- None of that, besides the first message has to be shown in chat, besides maybe a time limit on the auction.
What I mean is there should be a GUI for the auctions, like a chest with buttons, (like /shop or /ah) and thats where it would be held, not in chat