A reason not to buy used books

Today on the bus home, a white guy, around 50, was sitting a couple of seats away from me. He was reading a book when I first saw him. I didn't pay much attention to him until he sneezed many times in a row. He used a handkerchief to wipe off him nose and kept it in his hand after he was done. When the bus got to the destination, he stopped reading. I noticed that he closed the book and left the handkerchief between the pages he was reading. Apparently he was using it as a bookmark!