Re: What if people/AIM are selling/distributing powers?
I don't necessarily agree with your theory, but I'm starting to think Agents of SHIELD will probably serve as a way to introduce different kinds of Superpowers.
The Centipede already seemed to 'combine' the different explanations we've got until the moment: it's an Alien metal (possibly Chitauri), emits Gamma Radiation (The Hulk) and contained mix between the Super Soldier Serum from the forties (Captain America) and Extremis (Iron Man).
Franklin Hall will get his powers in Episode 3 (we saw an Inception-esque fight in the trailer) and probably as a result of an experiment.
I wouldn't be surprised if we get other poeple with powers though, Marvel seems to have adopted 'Gifted' as a way to call every super (which I imagine will include the mutants SW and QS).