Middle man is ok solution, but you have much more better one available, and it would not require anyone to monitor anything.
Just enable one generic npc to exchange zen for LDB ( example ) and vice versa, since everyone needs LDB.
That way, whoever sells something will be able to get LDB for whatever is selling to another player, and whoever is bying will be able to trade his zen for LDB.
No need for middle man, and no way to scam, since everything is done through regular trade.