For an unregistered .com. Most desirable .com's have been squatted, and the squatters want five to six digits for the domain. The business model deserves a fiery death, yet it thrives.
Additionally, as a web developer, when I see squatting on a domain I want, I will be absolutely livid. Give me a place to report squatters and I'll happily help you crowd-source what was previously a Hard Problem.
It could even be done through the registrars: for every year you hold onto a domain, the registrar withholds another 10% if/when you sell it. Legitimate companies and people wouldn't suffer, since they would want to keep their domains more or less forever. Those looking to flip domains would have to consider their diminishing returns.