Did he bother to get a necropsy? If he didn't, it's more than a little suspicious. If I had purchased a boa that I even *thought* might be a little sick, or fishy, I would have been in contact with you at least once a month, and upon death, would have gone right out for a necropsy to see what the problem was, to see if it was something I did wrong, or something that the baby had come with.
As the seller, if he didn't bother to get a necropsy done, there's really not a whole lot you can do. Offer him a discount on a future, similar baby perhaps, but really, you've done your part. You extended the guarantee, and he was suspicious, but not suspicious enough to really DO anything about it, aside from getting parasite medicine. Did he even have the vet test for parasites, or did they just preemptively treat it? If he had something going on like a bacterial infection in his digestive tract that was hindering his absorption of all the nutrients from his food, then stressing him out with medicine for something he didn't even have in the first place would just exacerbate the situation.
I had a similar issue with a baby
BCI that I purchased last year. She'd been eating and pooping, but not gaining ANY weight. At purchase, she was 90 grams, and 5 months later, eating once a week, she was 86 grams. We went to the vet, thought it might be parasites, but after a few swabs and bloodwork, it wasn't. We tried some broad spectrum antibiotics, but she stopped eating and died less than two weeks later. After the necropsy was performed, we discovered she had an absolutely GIANT cyst on her heart that she was most likely born with, and there was nothing that we could've done. If it was something congenital like that, there's really nothing anybody could have done, but if he didn't get the necropsy, you'll never know.
