I was actually just about to post about that as well. earlier when I was on my phone I couldn't see it clear enough.
The things I notice are:
1: The plastic bag. Why photograph the snake in there?
2: The rodent. Again, why? Did they just leave this snake in a deli cup with the rodent thinking it would eat? "Had not been fed" looks like a lie when there's a rodent in that photo. it was at least attempted.
3: no airholes. Unless they were in the lid... but also in a plastic bag? Still concerned about breathing
4: The only ACTUAL sign of something wrong... there is some discoloration along the belly of the snake that looks more green. This WOULD be signs of either severe necrosis or the body breaking down after death. Otherwise the snake seems posed like it's trying to get out...which makes me wonder if the green I'm seeing is actually just a lighting issue.
Either way, I'm sorry that this happened to you! Loosing one that you raised so soon after changing hands can hurt.
There is a little bit of fault on you for not catching the spam, but considering the amount of election spam ~I~ had been getting.... that's understandable.
From there.... unless you stated at the time of purchase, the average health guarantee for reptiles is typically VERY short. You're usually lucky to see anything past 24 hours. Some will give you 72 hours. Anything after that is pretty rare, I know one breeder who gave me a 30 day which shocked me. That said, some breeders will still listen to you on an issue past the guarantee and try to make things right.
But this is a super difficult case here.
Again, the missed message didn't help... but the photo they sent is not definitive in any way. Once I had the unfortunate DOA leopard gecko show up at my door. I had at least 4 different photos to send to the breeder as proof of death. Thankfully they responded quickly and I didn't have to freeze the little one. Without the extra proof, I don't know how far you would want to go.
Depending on how they paid can also be an issue as well. Cash? it's all in your decision. Credit? They may try a chargeback. Paypal business? Possible chargeback. Paypal F&F or Venmo will not allow chargebacks.
If there's a threat of chargeback, you may have to compromise on something like a partial refund. But honestly they did the bare minimum in contacting you/proof of death, so if this was my own animal I would be hesitant to do anything like a full refund or replacement.