Ball Python Cause of Death

[Malum Argenteum]

I of course don’t know if I can trust them

No, you really can't. It could have actually mysteriously died. But it could have been dropped, or taken outside in the cold, or been subject to any sort of neglect or abuse.

The only way to possibly know would be to have the snake necropsied. It might be (or might have been) interesting to offer to pay for the necropsy (either on their end, or send the snake back to you and you line up the necropsy) and see what the buyer says.

Are more photos available? Did the buyer save the body, or snap one photo and discard the snake? I ask because if they discarded the snake before getting to the bottom of things, then that's a red flag that there is some withholding of information going on.