Used tires are legal in most US states, and 30 million sell a year. Whether one is safe comes down to tread, age, repairs, and the damage you cannot see.
A practical checklist for judging a used tire: read the DOT date, measure tread with a coin, spot bad repairs, inspect the sidewalls, find punctures, and check for recalls.
The US scraps about 264 million tires a year. From tire fires to 6PPD-quinone in salmon streams, some of the scary numbers hold up and some are junk, and reuse still helps.