AWS reserved marketplace?

Security wise it's no different than any other instance; specific machines aren't associated with the party who originally reserved the capacity in any way.

You're right. The deals businesses do at this level preclude you from selling the resources you've purchased at a discount to another party.

Yes there are marketplaces for this with some providers like AWS. You are buying capacity (like a voucher) for a server not a particular server.

Cycle Computing does this. ATLAS, an LHC experiment, recently demonstrated this. I even built a product at Google that made this possible. We used Native Client (users compiled their binaries using the NaCl toolchain) as one part of the security container.