As owner of an exclusive right, in this case a patent, you have the right to do whatever you consider. Patents may be licensed and get economic compensations for the use but keep the ownership. And, you can also sell them.
You need to take into account that, from the moment the transaction is done, the buyer becomes the new proprietor of the patent y, therefore, the new owner of the exclusive right of exploitation. From then on, you no longer have any rights over your creation.
License contracts may have a specific duration, but selling a patent is definite and irrevocable.
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