Again, when you buy games, you don't buy property like a house or a car. You buy the use of someone else's intellectual property. It's not fully yours, for example it's illegal to copy it and sell it to someone else. So long as you get it on a physical medium, you can resell that medium. If all they sell you is a license, like digital distribution, then you can't resell the license. That's all there's to it.