I believe that when photographers put their work with agencies such as Getty or Alamy or whoever that they still retain the right to sell their own work.
The agency works purely as an agent retaining a percentage of the price at which they they sell prints and passing on the balance to the photographer.

What puzzles me is If copyright runs out 70 years after the death of the photographer. How can anyone claim to own the copyright of Sharp's pictures? You might be in possession of the original negatives but surely you can't claim ownership of something that has expired.