The easier it is for a customer to find something, the more likely it will sell. That means that items that would not normally sell well offline might sell online. That's the reasoning behind the so-called "long tail" principle.
In most businesses, 20% of the products account for 80% of the sales. Consequently, there is little incentive for retailers to stock items that sell very infrequently. On the other hand, people who want to buy these more obscure items can shop online. Searching online is easier, so in fact it may be that most of the sales of online stores come from obscure items that sell rarely. This is the idea behind the so-called "long tail effect".
See also Brynjolfsson, Erik, Hu, Yu Jeffrey and Simester, Duncan, "Goodbye Pareto Principle, Hello Long Tail: The Effect of Search Costs on the Concentration of Product Sales" (November 2007). Available at SSRN: http://ssrn.com/abstract=953587
