I've bought a couple in the past, I also take the view;
So if you brought a second hand car would you change all for tyres? As you don't know their history.
They could be premium brand tyres with plenty of tread but that's not to say the nearside tyres haven't been run over a kerb at 30mph. How often do you hear of car tyres suddenly disintegrating and causing an accident? Sometimes you find the tread detaches from the tyre but that is usually under-inflation or old age.
First I bought was from some dodgy old backstreet garage, which was for use as a spare tyre. Should've walked away but I needed to replace my spare. The guy wouldn't let me roll the tyres out from the rack to look at the sidewalls (I did anyway) and then after fitting the tyre, decided to tell me the balancing machine was broken. Won't be going back there.
Second one, I needed a spare wheel and tyre for my van. Wheel was going to cost £30 for a rusty old rim, plus £70 for a new tyre. Managed to get a good condition rim with a part worn tyre with 9mm thread for £60 from a breakers. The tyre had a tiny flat spot so it had obviously been skidded when the van it was fitted to had been written off, but the rest of the tyre looked perfect and no damage to the rim.