Exactly this, and for #2 I would have the new owner sign a first right-of-refusal contract where if the new owner needs to rehome it then I'm first in line to get it back at no charge. There would be a rather steep fee for violating it too. Otherwise you know there are people out there who would take that free snake and flip it within a few days, with no disclosure of the deformity.
OP: I would put feedback threads up here and on the FaunaClassifieds.com Board of Inquiry on the seller. Whether or not the critter is eating, an underbite is a defect that should be disclosed.
It's not stupid or irrational to worry about quality of life and potential problems when keeping an animal with a defect. Anyone who breeds has to consider them before they start throwing males in with females.
I would just monitor her weight and growth, and when she gets up to adult size if she does stop eating just weigh her every few weeks to make sure she's not dropping faster than most other BP's, as it's not unusual for them to go on months long fasts.