Actually it's interesting task to come up with creative way to convert amazon credit to hard cash (that does not expire).

I don't think bitcoin mining via CPU (virtualized especially) is feasible for that anymore.

Altcoin mining on the amazon gpu machines was still viable, last I checked.

Well, it WAS, until Scrypt ASICs came out about a year ago. Now it's not viable. You'd be lucky to get a percent back.

Would running a PaaS on top of EC2 be considered "reselling"?

Only if it is purely a subset of EC2.