In a few years all those old phones are upgraded and sold, and the owner wants them clean.
Android 4.4 The newest version that they did not test. Is from October 2013. And I know that OME's takes some time to switch to new versions of Android.
But lets say that the last was switch one year later. So October 2014. That makes the phone 8 month old by now. And it was not a High End device.
So in a few years. Then it's a really old device. So how would buy it?
I can understand people want to sell there old phone. But a old Smartphone is OLD!!! And you can get new low/mid range devices pretty cheap