I think informed consent is key - and I know I would give mine for the right sum, unless a family member called first dibs. I am planning to be cremated and a few ounces of ashes more or less won’t make a difference.
I can hardly see any other way to obtain a human skull ethically. If the seller is honest, they should make the signed consent form available to the buyer upon request.
They might be at ‘evangelically’ child free folks.