Maybe where you live but there are plenty of places in Missouri that sell used tires. However, if you’re suggesting it’s illegal to sell used tires as new, then that would undoubtedly be correct.
Whoops, I stand corrected about the illegality of selling used tires. A search turned up a number of ads from used tire sellers, including some in my most nanny of nanny states. I’d been told by sales people that there were rules against it, but perhaps this was just their attorneys speaking.
That isn’t to say that mounting one with an unknown history is without risk, as some types of damage may be difficult or require specialized equipment to detect initially.
About every tire dealer around has a rack of used tires. You aren’t going to get four matched ones but if you need one . . . I was hauling a new dock to our cabin and got about 5 miles away and blew a tire. Put the spare on and drove to the local station. They had a used one for $20. I was 200 miles from home and was close to putting a new set on anyway. So that got me back home again and a couple weeks until new ones were put on. Who better than a tire shop to check a used tire out before they put it on the rack? There is risk in everything so you have to decide what is undue risk.