零售 - Why is the character for "zero" used in the Chinese word for retail?
零 means "a part (fraction,fragment) of something" here. Some examples for this meaning:


  零钱 small change such as RMB1, 2 or 5 (depending on how much you are talking);  It also has another meaning. 
  I.e,you buy something at a total price of 8.3,and you give one note of 100 to the cashier, so the cashier needs to return you 91.7. in Chinese,we also call that 91.7 零钱(here,零 refers to fragment) . Usually,the cashier may ask you"您有零钱(here,零 refers to small amount)吗？" Here,he hopes that you have notes with a small face value.In this case, it would be 10 so that he doesn't need to return you too many notes.
  
  零件 part (in a machine) as a "part" is a part of a component in a machine.


Why is “零售” used?

Well, 售 means "to sell". We have another word opposite to "零售"; it's “批发 (wholesale)”. "批" means "a batch", "a package", and "发" means "to distribute, to sell".

The wholesalers (批发商) will buy a lot ("a lot" refers to 批 here)of goods from the manufacturers and sell them to different retailers(零售商). The retailers sell goods to end customers and generally a customer would only buy small amount (a small fraction, this is why we use 零 here) of the goods. You won't buy 10 soaps at a time, will you?