In such cases, the 'victim' would - presumably - have recourse to refund/compensation/rectification. Unlikely in a private sale.
The best way to stop such cars falling into the hands of unsuspecting new owners is - in my opinion at least - to try to stop them being built that way in the first place.
To answer t your second point first - just how do you propose to do that - wear your gilet jaune outside their house and garage to prevent parts arriving and the work progressing ? C'mon....
As to the first.....you can't be that naive today surely ?
One specialist declared bankruptcy and then mushroomed up somewhere else, another threatened the owner with a lawsuit, another seemingly washed their hands of the car leaving the owner to solve the issues and Ian Patmore had some strong feelings on the UKs' Z engine builders I remember.
And this club doesn't want to hear of 'the bad-bits' - even you have commented on the 'happy-slappy' attitude so frankly, it's down to the person opposite be they private or business......the 'law' is an expensive luxury for those who can afford it.