Yes you can buy a share but can you afford to pay if the engine throws a conrod and needs a overhaul ? A scary proposition no one ever mentions on this idea of buying a share and hour building with your shared plane.
One day you have a 10 000 quid share in a perfectly servicable plane, the next day you how 2500 more to overhaul the engine ! I would rather hire some one elses plane and let them have that risk.