What's the effect of the used car market on car price? While justifying his decision to sell a one-use textbook to his students, a BYU econ professor states:


  A word about the Kearl text. I do not receive any royalties and have
  no economic interest in the text – when you buy the text, not a single
  penny goes to me. It’s designed to be a “disposable,” one-use text in
  order to keep the price down. (The used book market actually drives up
  the price of texts – to see why, think of what would happen to car
  prices if used car sales were not permitted.)



How does the used car markets drive up the price of the good? With an used market, the buyer is willing to pay more, knowing that he can resell later. However, without an used market, everyone is forced to buy new cars, raising demand, and thus raising the price of new cars. How to think about these countervailing effects?
Are the used books and used cars market actually analogous like the professor suggests? The professor and his publisher have a monopoly on the new textbook, but no one controls the new car market. Therefore, if people keep reselling text, the publisher will use their monopoly and raise the price of new books to compensate. In contrast, car manufacturers can't do so due to competitive pressure.
OP, 

imagine that there are two kinds of people, rich and poor, and no market for used books. Rich people are willing to pay more for new textbooks. Poor people cannot afford to buy new textbooks. In the absence of a market for used books, poor people will not buy textbooks. 

Imagine now that there is market for used books. Poor people are now able and willing to pay second hand textbooks. And rich people now have someone to sell the books too once they are done using these books. 

A market for second hand textbooks raises the price of textbooks because some people are not able and willing to buy new textbooks but are willing to buy second hand textbooks. In the absence of such a market, they'll spend their income on other goods that they deem more important.

Think of it otherwise this way. In the absence of a market for used textbooks the full value of a textbook is not exploited because some people who would wish to trade with one another cannot trade with one another. 

Hope that this helps.