That doesn’t make sense. You always give the carte grise to the buyer, so that they can register the change of ownership. There are no circumstances when the seller sends the cg straight back to the prefecture. If the car is scrapped the carte grise is given to the vehicle dismantler, who has to be registered as such, and he has special forms that he uses to acquire the vehicle from the seller and register its disposal. If you sell to anyone else you give them the carte grise, with the certificat de cession, so they can do a change of ownership. Legally speaking you haven’t been able to sell a car for parts for around 10 years, I know people still write “pour pièces” on the cg but it has no legal significance and the prefecture will simply ignore it. Either you sell a car or you don’t, and if you sell it then the car and its paperwork become the property of the new owner, but in this case the seller seems to thought it was OK to take money for a car but not allow the new owner to take legal ownership of it. Is this a chain of Brits? Two buyers in a row who weren’t aware of the implications of “buying” a car without the correct paperwork, and a seller who didn’t know the process either - it has to be, doesn’t it?
Basically the issue is that you can’t leapfrog over owners. The prefecture isn’t interested in renovations or anything else, all it is interested in is seeing an unbroken chain of ownership, with each attestation de cession signed by the person whose name is on the carte grise who is also the person named on the previous cert de cession, so that (a) it proves the person who sold the car was the legitimate owner and had the right to sell it and (b) it ensures that the registration tax is collected each and every time the car changes hands. Currently, registration tax is due from the person who bought it and did it up but never did the change of ownership, and unless they start falsifying paperwork and pretending this person never owned the car, that will have to be paid before it can change ownership again.