You're right that it works better on a macro level but under the assumption that the clerk's ignorance goes unpunished: how is happiness in the population not increased through the sale of a prized commodity to a buyer that derives joy from it?
First I think it's a pretty large assumption that the clerk's actions will go unnoticed. Someone along the chain of managers (from department to store to region and so on) Is going to notice a register override for $320. And then blame is going to be kicked as far down the chain of command as it can. Someone in this thread already stated that a memo about this is already being distributed through Wal-Mart.
But setting that aside, the missing value has to be absorbed somewhere. Someone, somewhere will feel the financial effects of this action. It will probably be spread across multiple people. From a Utilitarian perspective, something like a PS4 is a luxury good. It's value can only be extracted once lower level needs are met. A starving family would much rather have $400 worth of food for instance.
If the people negatively affected by the PS4 sale lose some level of access to more primary needs (food, shelter, and the like) so that you can have a more enjoyable leisure time, then the act does not serve the greater good. Also because this act is part of a much larger "Taking advantage of a Corporation is justifiable" sentiment, the way that this act would encourage future similar acts must be taken into account.
And, if you already have a console, say a 360 or a PS3, then all you are gaining is a slight increase in good. The delta between your enjoyment of what you have and what you would gain.
All of this speculation involves really small amounts of something that is almost impossible to quantify to begin with, but that's par for the course in discussions about Utilitarianism.
Honestly, the whole thing is a messy, murky exercise. Which is why my default assumption is that people are naturally selfish and will attempt to justify their actions. The concept of The Greater Good is a great philosophical thicket to hide in.