I like how you can have multiple Linkshells active in FFXIV vs XI and that got me thinking.
What if someone created a 'Buy/Sell' Linkshell and got everyone on the server to join. Then people just used that linkshell to 'shout' for stuff they want to buy and sell.
You could have it equipped or not depending on whether you were looking to buy or sell something or you could just leave it on all the time and scan the offers coming through.
Ok, in practice, it would probably be a nightmare. But in theory.... I mean seems plausible.