I ask again, did you not see pictures before you argeed to the trade? I agree that he may have sold you sick snakes but there is no way to truely prove that. He agreed to pay the vet bills even though you can't prove they were sick with you got them. To me he sounds like a pretty stand up guy and you are an angry customer with buyers remorse because you didn't think the trade through well enough.
If that's what you think the purpose of quarantine is then you honestly don't understand it. Yes quarantine can protect your new additions from anything your collection has but the primary purpose of quarantine is to protect your established collection from the new additions. You expressed a concern that the snakes you've had for a while might become sick because of this, and this is why you quarantine.