If a company buys a patent, they can license it to third parties but there is a cut-off at the date of purchase to sue others for infringement, so that if someone is using it unlicensed before that date they cannot be sued.
They would then have a 'cooling-off' period, after this date, in which time the new patent owner can inform patent users of the new licensing conditions. There is a time limit for those not meeting the license conditions to either stop using the patent, or license it. If the patent owner does not inform those infringing the paten within the cooling off period, then they have missed their chance and cannot sue. If the infringer is informed, and does not meet the licensing terms within their time limit, then they can be sued.
Such license terms should be fair and reasonable, and potentially liable to oversight by the courts to ensure they are not extortionate.
This should cover both legitimate patent ownership and transfer, and remove the fear from others of being sued by trolls.
edit - This should only cover the case where ownership of patens changes hands. Those originally registering a patent and not transferring it should continue to be protected against unauthorised use.