minecraft.novylen.net

http://dev.bukkit.org/server-mods/shopkeepers/

It might be complicated for new players, but you don't need both players online for a trade.

There does seem to be a large barrier-to-entry for new players and barter systems. It's also a bit of a headache for moderation, as theft is a little easier, either that or nobody trades because they don't want to get ripped off.