Is all used cars sold as is?

Is all used cars sold as is?
can someone tell me is all used car sold as is or do you have to put in writting as my son sold his car which he owned for 3 yrs i had a lot of work done to it as i didnt want him driving an anreliable car know the buyer wants his money back as this is the first thing my son has sold he never wrote a receipt or any paper work at all


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