The Offer unit sometimes does this – comes up with random crap from ages ago. The foreclosed house is common.
I would suggest first seeing if they can even get that information from the Dept of Motor Vehicles in some simple way, like online or with a phone call. If not, then perhaps an affidavit by them that those cars were wrecked/disposed of years ago and are no longer in their or another family members possession. Have them sign it at a bank in front of a notary. I would also get their auto insurance page that shows what cars they actually have insured. That should suffice. Finally, a printout from DMV of the cars they do have registered would help too, but as a last resort. I hate going to the DMV and will do anything to avoid it at all costs.