This walks a line between helping the customer to make decisions manipulation (I'm being generous here). If I saw an online store doing this I would think less of them (again, being generous in my language) and go somewhere else. Also I can tell the difference between real scarcity (like an eBay auction timer out) and simulated scarcity...
... is what I like to think happens. Obviously it works and the chances of me, or anyone else who thinks the same way, being exceptional is probably slim.