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.
Admin Store
Re: Admin Store
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.
"That line you read, of course is mine, however, read so badly, it sounds like yours" -- Roman Noble