I roleplay, so I like the game to feel as real as possible. Money is supposed to change hands when a transaction takes place. That means one person has more money then the other person afterwards. A merchant who always has 400 gold after every transaction is as lame in my view as an an archer who never runs out of arrows or a mage who never runs out of mana.
Besides the roleplaying factor there is the gameplay factor. Gold is already way too easy to come by in the vanilla game. In no time at all most characters have more gold than they know what to do with. Giving merchants "magical gold" that never ever depletes feels like cheating to me.
And also, I miss the tiny bit of strategy (it's not much, but I'll take what I can get) involved in finding merchants who will buy my goods.