I'm also in agreement with the majority here..
When you opted for local pickup versus shipping them home, at this point you took on the responsibility of the animals, then you decided to pick them up at the begining of the trip versus on the way back home.
even if the animals don't seem to be stressed, this prolonged transport (even if you did everything in your power to care for them) was a bad decision and caused that animals life.
it's always sad to lose an animal

but you have to sit back and ask yourself, WHY should he even compensate on the lost ??? of course it's always nice when a seller does, in the end this is a buisiness transaction and you received two healthy animals, one dies days later in your care while in their temp enclosures on a prolonged trip..
at some point, sellers can't always be giving out refunds because someone managed to get their animal killed. sad it is and i feel for you...
I don't even understand why you wouldn't purchase again from him..as this death is in no way his fault..
when I sell CBB emerald tree boas(or any other snake) the ones I ship, i always guaranty live arrival and hold on to the $$ until i contact the new owner a few days later to make sure everything is perfect.. I would offer a refund if anything was wrong and have the animal shipped back, or partial if they decide to keep it..
but after a few days/weeks animals that require a specific humidity and gradiant heat are way more delicate than other species, sellers can't be held responsible for someones lack of proper care.. i know you did everything possible , but in the end, these animals should have been set up in their forever homes asap and not go on a road trip in tubs..
both of you are acting like pros in the situation, unless you have a necropsy done by a vet to test for a cause of death, you could ask to contcat the other owners about theirs. but if tests show nothing, that the others and yours are doing well. that both of yours looked great,same size and ate plus the fact he had them cbb since born !!!!??? the only probable cause of death is stress related due to improper conditions for a extended time..no matter how much you think the conditions were perfect, clearly this animal did not..
sorry for your lost but I see no reason why a refund or even a partial one should be given..the animal might still be alive today had you picked them up on your way back or had them shipped..