Do we have to give them a refund?
Unequivocally, NO.
All private seller to private buyer used car sales are "as is", unless a warranty is given in WRITING.
All those two dummies bought was a 15 year old used car from a private citizen, as private citizens.
That said, did you follow MI law as regards title transfer?
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When selling your vehicle, accompany the purchaser of the vehicle to a secretary of state branch office to assure the title is transferred into your purchaser's name.
If you cannot do this, maintain a record of the sale for not less than 18 months.
A "record of the sale" can be either a photocopy of the reassigned title or a form or document that includes the year, make, vehicle identification number, name, address, driver license number, and signature of the person to whom the vehicle is sold and the purchase price and date of sale of the vehicle.
Under Michigan law, a seller is not liable for any damages or a violation of law that subsequently results from the use of ownership of the vehicle by the purchaser if the owner maintains proof of sale as noted above.
As seller, complete your part on the title assignment by entering:
The vehicle mileage
The selling date
The selling price
Your signature
Note: If there is more than one owner named on the front of the title, all owners must sign.
Examine your title for any liens (bank loans). If there is a lien it must be released by the lienholder in the appropriate signature space, or you may attach a lien termination statement from the financial institution.
Make sure ALL buyers enter name, address, and signature in the buyer's section of the title assignment.
Do not make any changes on the title.
Any alterations will void it!
This includes erasing, scratching out, use of white-out, etc.
Important:
Keep your license plate! Never leave it on the vehicle or let the buyer borrow it. You can transfer the plate to another vehicle you may own or purchase.
The buyer may drive the vehicle home without a license plate using the most direct route if they carry the assigned title from the seller.
Cancel your insurance coverage on the vehicle sold.
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Michigan SOS Paperwork When Buying a Car | DMV.org
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SOS - Don't get taken for a ride when buying a car
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SOS - Seller's Responsibilities - Selling a Vehicle
