Part worn tyres are of varying degrees.
Some come from Germany due to the 3mm wear limit.
Some come from accident damaged cars.
Some come from cars where the owners change them to a preferred make rather than whats supplied.
Whats required if you want to use them is that you assess the risk and, like Mark, think there should be a universal testing standard so that they can be properly tested and graded.
When I ran my Cavalier TD - from 6 months old until I scrapped it withalmost 200K miles on - I used to use part worns at times. Back then, two young kids etc etc money was quite tight. I was choosy though. The new panda and other cars came fitted with, as I recall, Michelins, from the factory. The fleet ran on Goodyears. Result was that Tyre Services/National Tyres were contracted to change all the tyres and I then purchased a few of these 'part worns' from them afterwards. The wear was, literally, factory delivery miles only.
So, there are degrees of wear out there.....