Aw thats so sad
Ok well I think I know the problem.
If you had everything 100% correct, it would'nt of been your fault. And the breeder/owner just proved that by saying "not my problem" rather than trying to help a bit or give you advise/solve the problem. He obviously doesn't really care about animals and gave you a sick lizard just for money without telling you he was ill so you could get some help (since most people probably wouldnt of bought him for the full price).
If he was 100% trustworthy with having a healthy pet for sale, I know this may sound strange(i thought it was at first too), he wouldve given you some sort of warranty to prove he wouldnt pass away at any point. My sister's pug had a warranty too from the breeder lol its the weirdest thing I ever heard at first, but they said they would refund if the pug died for some reason, proving that they were selling a healthy puppy.
A seller who cares about their animals, wouldve tried to help out 100% because yeah, it's a sad thing to happen. It's not like its a $2 peice of plastic you just snapped, its a living thing. & also 8 days?! thats very short. Even wild frillys in remote areas who are short of food and shelter still make it through longer than that! It was seriously the seller's fault who just wanted to con you.
I'm sorry you had to go through that, I can't think of anything you can do about it, so I'd say next time you go to a more known/trusted breeder who can guarantee you your new reptile will live under your good conditions, otherwise your money back. It's not worth losing another little guy
Anyway I hope that gave you kind of a clearer picture! & not to think it's something that might've happened because of you.
M.