It's not very hard to securely run a virtual machine that could offer clock cycles from your CPU or GPU to some enterprise that needs them.
It is simple to secure a compute node against running malicious code, but not at all straightforward to secure a customer against a malicious compute node. At least not without huge overhead.
A marketplace for 'compute cycle futures' is also subject to default risk if the compute node cashes out and never delivers, so they will have to be bonded (or something equivalent).
But despite these challenges, I think this would be a great direction.