Books are commodities in the sense that there is a market for them, just like everything else.
Plain and simple: People buy things when they want or need them. And people sell things when they would rather have the money.
This is true for all economic goods. (regardless of what laws are put in place).
A rule of the ages is that when people need money they sell what they have on hand. Thousands of booksellers have profited by this fact.
-Larry McMurtry, Literary Life