Some people work for a living, some make money buying futures, some speculate in stocks (not too successfully of late...) and some can actually make money selling used books.
From Sign On San Diego, here's the story of one such individual, Nancy McReady, who scouts the Friends of the Library sales and is "on the lookout for the occasional gold mine, such as the worn coffee-table book [she] found at the Vista branch library for 50 cents and resold for $250".
“You have to research,” McCready said. “You have to know what you're looking for. You have to develop an intuition.
Take it from birdie friends/Friends, this woman is on to something. Price your books appropriately and do your homework. The days of the 50 cents book are (or at least should be) over. Utilize those donations to make real money for your library.