Scarcity

Theory:

Increase the (perceived) scarcity of something and it's value will rise. 

Application:

Can you make your products look more scarce? People don't like missing a good deal or product and you can give them an extra push by making a product look scarce. Prevent people from holding of any longer from buying from you. For example, you could show a discount-countdown-timer or a 'almost out of stock' notification on your products.