How do online sellers like flipkart offer heavy discounts on festive deals?

That might sound little interesting with a wink, isn't it? But it is a fact that companies selling their products online through web marketing portals like flipkart, snapdeal, amazon, naaptol etc. often offer more than 50% discount on almost all deals. The question is, how, do these companies provide or afford huge amount of discounts or concessions via online selling platforms?