I fully acknowledge that it's a grey area, but I've personally always considered resale of digital goods (goods which can be obtained purely digitally, even if sold in a physical medium) to be unethical, although legal. If I'm going to pay money to it, I want the money to go to the person who created it, not to someone else who happened to purchase it or, worse, some company that provides no value other than encouraging those transactions.

To me, resale on physical goods is ethical because there are two core differences with those which could be acquired purely digitally. Physical goods degrade with use, providing reduced value compared to new goods. And it is better for unwanted physical goods to continue to provide value for someone than for it to enter a landfill.