The mistake is in thinking that domain names should be property. They're not property, they're address space. Imagine if trademarks worked this way -- right now to get a trademark you have to actually use it (or certify that you intend to use it), you can't just squat on a thousand different names and hope somebody is eventually willing to pay you for one of them.

And the solution should be the same. You can't sell a trademark. What you can do is sell the goodwill it has accrued and transfer the mark along with it. That sounds like the same thing, right? But if you haven't used it there is no goodwill -- in fact, if you stop using a trademark then you eventually lose the rights to it. It shuts down the squatters but not anyone else, which is why we don't have trademark squatters the same as we do domain name squatters.