to a point i would agree stew, but you have to be very careful what you put on, i would want to individually inspect each tyre before it went near my car.
i have seen some part worns that were apparently sold as A1 top condition yes it was a top brand name tyre and yes it had about 5mm tread, but there was alot of repairs in them and one or 2 i would have said were way to close to the edge.
my advice would be if you go this route to be extremely vigiliant about what is fitted.