I think the question is why not sell them openly instead sell them via backgate..
That people sell food and houses is disconcerting in the physical world and creates real problems for real people where some can't afford to eat or have a roof over their head despite a global abundance of resources. That people do the same in the virtual world, with literal numbers, is beyond the scope of comprehension: pure madness.
Be dismissive all you want.
Why should IP addresses be private property? Why not simply reclaim unused IP addresses instead, and re-distribute it using the usual mechanism?