I'd go back and talk to them. If they sold you a used crank that is fine. But I would expect that they would have checked the used crank to be sure it had good condition threads. Assuming that this SHOULD have been the case you should at least be able to meet them halfway on another crank arm. And while you're at it talk to them about why you've had two crank arms go bad on you in so short a time. Generally this sort of thing is pretty solid and it's rare to have a crank arm strip out from riding. If they re-used your old pedal perhaps the thread on it is slightly undersized for some oddball reason?