Caporegime
Every car you buy private is sold as seen. Just check the tyre is ok, end of the day what is the difference from buying a tyre from a car that could possible be from someone buying alloys and selling the tyres on? You buy a second had car you have part worn tyres. Loads of people on here are on a very high horse, check for puncture repairs or any signs of uneven wear and what could possible go wrong? If its come off a crashed car you have to ask is it only because of a dodgy tyre? Many people crash because they don't asses the road correctly and are travelling too fast, or maybe somebody crashed into the back of you, your car is written off and sell the tyres on?
Nothing makes sense, so you buy a second hand car give it a through checking over buy it then replace all 4 tyres for new ones?? I highly doubt it.
I don't buy part worns, but can see a market for them. The OP should have checked they tyre prior to having it out on his car. Putting a tyre on a car with a chunk missing is never a good idea, if mine was like the OP it would get replaced.
The difference between buying a car with part worn tyres and simply buying some part worn tyres has already been covered in this thread.
The prospect of driving around on a set of tyres which has lived it's life on a car which you can inspect for damage, asses its overall condition and question the owner, is somewhat different from buying a set of tyres which have come from an unknown car, which you've never seen, you don't know why they are now available for sale, you don't know how they have been stored, nor do you know the competency of the person passing them as ok.
Totally different situation, buying part worn tyres represents a much bigger unknown than those which come on a secondhand car.