Definitely rude behavior on the part of the host to want to wring every last penny out of his rental before selling, but as Andrew says, no one can force the host to let you stay.
Even though this place has sold or is in the process of being sold, if you have a opportunity to leave a review (like the host doesn't delist the place right away or has other listings under his profile), make sure to say what happened your review. Reviews are linked to the host's profile, not the individual listing, so if the host has other rentals, or opens one in the future, at least future guests will be warned what this host has done this to you.
@Sophie1663