Hi,
Nobody should be selling sick animals. But mouthrot is fairly easy to spot before it gets to the stage of killing a snake through the infection so they should have seen it.
Then again you should have contacted the store as soon as you noticed something was wrong - some stores would have either replaced the animal or have had a vet recommendation and be willing to help with the cost.
But I can see where they are coming from with the receipt thing - you would face the same issue with any other purchase.
If you paid by credit card or bank card etc that might be acceptable to them as proof of purpose if you take in a statement showing the charge.
Basically all they are looking for is some form of proof of purchase - they might have 2000 reciepts from that day and, unless it specifically said " for a boa" or didn't have any other items to change the total payable at the bottom they would have a heck of a time finding it with any certainty. Your card details would let them find it fairly easily though.
That was also behind the request for the snakes body, it wasn't because they doubted it had died, in all likelyhood it was just proof that you had bought a boa.
**edit** Dang I type slow **end edit**
dr del