I've never heard of anyone buying excess cloud capacity from a private party. Is this a thing? Is there a market for this? Sounds like a security nightmare.
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.
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.
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.