What's the effect of the used car market on car price?
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.