In this scheme, you should theoretically be able to implement transactional protocols (two phase commit and friends, see, e.g., http://dl.acm.org/citation.cfm?id=3027893) that ensure you do not actually sell two people the same thing, but may return an error at buy time that the inventory disappeared while you tried to buy it.

You pay a sometimes-very-high cost in throughput.

Of course, talking about mongodb, it's probably broken in a myriad of ways no matter what you do :)

I'd like two of this product? Increment reserved in Mongo atomically, fire a bus message that inventory was reserved. I'd now like just one? Again atomically remove one from reserved and fire another message. Simplified of course.

Two users can't reserve the same inventory, reserving the inventory is atomic, and everyone else occurs during reconciliation. Typical distributed systems.

You have to handle over-selling no matter what. The "source-of-truth" of you inventory system is whatever is actually, physically sitting in a warehouse somewhere. Not what your database says.