Former licensed home inspector and current 20 year commercial construction project manager here (Purdue Construction Management 😎)
A lot of sound advice already here, I’ve got a couple other thoughts to add -
These homeowners sound like absolute idiots. Who shuts off all their smoke detectors and leaves them that way over some dead batteries when they are in the process of selling their house, not to mention, the serious safety risk to your family? I would scrutinize everything they tell you, they don’t sound very trustworthy.
Did your inspector word the report that a licensed electrician should investigate this (he should have)? A lot of people will try to get away with “doing stuff themselves” when selling a home to save money, but the major components of the home (electrical, plumbing, hvac, structural, etc) should only be addressed by qualified/licensed professionals, and the seller should be providing receipts showing that they have paid someone to do the work. Don’t let them just say they took care of something like this themselves if there is documentation that a professional should be doing it.
This may not be a big deal at all, and these sellers may just be knuckleheads, but don’t be afraid to walk away. You can always find a better house. Trust your gut. Also, don’t be afraid to get another opinion from an unbiased realtor or home inspector.