Surely a 2nd hand tyre dealer is selling otherwise good tyres off scrap cars, just like any other 2nd hand parts. He's not going to sell worn out or damaged tyres as no one would buy em and no (reputable) fitter would fit em
Edit: quite apart from potentially saving money, re-using rather that throwing away perfectly good items is environmentally a good thing too
I'd agree that trying to re-use pefectly serviceable items is environmentally a good thing, particularly with goods that have a high environmental cost to manufacture such as tyres, if it can be done safely and cost effectively.
I'm unconvinced that most used tyre dealers have the level of expertise needed to determine whether tyres are serviceable or not, that the history of most used tyres is known well enough to assist that decision, that reputable fitters are commonplace in the second hand tyre industry, or that most of them really care.
Moreover I'm not sure a lot of used tyres really represent a money saving given the level of tread a lot of them have.