Just come out with a new edition each year where you move the problems around so that people have to buy the new textbook and cannot buy used. And in the occasional case where that does not work, change the title of the book occasionally so that students do not know which second hand book to buy.
Just come out with a new edition each year where you move the problems around so that people have to buy the new textbook and cannot buy used. And in the occasional case where that does not work, change the title of the book occasionally so that students do not know which second hand book to buy.