1.x/Trader
From Citizens Wiki
< 1.x
Traders allow trading of items for virtual currency. The trader can both buy and sell items to the player, and uses an intuitive interface to facilitate this. Warning: may be exploitable under lag until bukkit creates inventory events.
Commands
FAQ
Why does my trader let players dupe items?
This is due to lag and is a bug with Citizens/Bukkit. Citizens requires inventory events (that's programmer-speak), which Bukkit is not close to implementing.
Why won't you add Spout support so traders aren't buggy?
Spout is a great API for client-side additions to Bukkit. However, we are not interested in supporting it server-side. We don't want people to feel forced to download Spout if they want to use trader NPCs.