Found this post from before last semester, hope this can maybe help. Perhaps ask the professor if you can have the 9th?
Variety of Books for Sale!!Original Post: Tue 8/4/2015 at 12:58 PM
3) MACROECONOMICS (9th edition) by William Boyes and Micheal Melvin - $70
Call/Text anytime (253)279-2795 Mexico/Columbia area.