Good, unmarked, un-highlighted (un-read?) college textbooks can deliver fantastic profits.... if you mind your P's & Q's on Amazon. Here are the policies Amazon requires textbook sellers to follow. They've tightened up regarding books that come with CD's and/or online passcodes.
Here is a link to the policies page: http://www.amazon.com/gp/help/customer/display.html?&nodeId=200267740
Pay special attention to the prohibited textbook items at the bottom of the page.
Yes, I've often discovered good quality textbooks at yard sales and auctions that I've purchased for as little as 50 cents and resold quickly for $25-$50, sometimes even more. But I've also brought home textbooks that wind up being virtually worthless because I overlooked poor condition on inside text pages (underlining or highlighting) or I've become the proud owner of outdated editions.
Oh, well... live and learn. And, that's exactly who I am glad you're keeping up with my blog and researching the "Do's and Don'ts" before you run afoul of Amazon's rules.