"If I said you could not have it, you would want it more"

We value items that we think are in short supply think Amazon's `Deals of the day' and % claimed. 

Sites offering member-only flash sales with limited stock (Groupon, Achica, Wowcher to name a few) and those using a countdown clock think eBay and bidding wars, subconsciously persuade us into thinking we want and need that product. After all, we don't want to miss out, right? 

Based on Robert Cialdini's scarcity principle.