And yes, if the chometz is destroyed during Pesach it is the new owner’s responsibility. He has to pay up, and we can absolutely sue him for it. That is why insurance was invented. The insurance policy transfers to the new owner, so if the stock is destroyed the insurance will pay him, and he can pay us. We can even put in the contract (though I haven’t noticed it done) that in case the stock is damaged before it can be appraised we will accept the insurance payout in lieu of an appraisal.
Also, if there’s an Arev Kablan then instead of suing the buyer in court we can sue the AK in beis din, where we know the verdict will go to us.