More to the point, as-far-as-I-know this activity is monitored for, and using promotional credits for these purposes will invalidate your credits and you'll receive a large bill.
Source: A previous employer got a notification that our instances had been flagged as mining cryptocurrency, and they wanted to invalidate a low-six-figure amount of credits, and back-bill. Note that no cryptocurrency was actually being mined, and I don't know what heuristic they got caught in (although I have suspicions).
Maybe in the order of $50 rough estimate, that is 2 orders of magnitude less.
Bitcoin is designed to cost the price of electricity when mining on dedicated ASIC miners.
AWS costs are covering enterprise hardware with extra services, electricity is not one tenth of that, and AWS is using general purpose CPU (maybe GPU instances) which are not one tenth as efficient as ASIC.
But 5k in credits is worthless to someone who doesn't need them, so even if it's 1c, that's something. (Or really, so long as it's more than you can sell the account for.)
IIRC credits are non-transferable for exactly this reason, and also why billing information is required (even if you have credit). "Selling" a full account with credits (or access to that account to use the credits) would be super risky since it's still tied back to you, you will definitely be selling for less than the credit amount, and you would be on the hook if AWS invalidated those credits after-the-fact.
Source: A previous employer got a notification that our instances had been flagged as mining cryptocurrency, and they wanted to invalidate a low-six-figure amount of credits, and back-bill. Note that no cryptocurrency was actually being mined, and I don't know what heuristic they got caught in (although I have suspicions).