I have honestly never been to a used tire shop because the concept confuses me.
Sometimes part of a set is EOL and the others are good. If owner buys a full set, the good ones end up at the used tire shop (UTS).
Some brands have a satisfaction guarantee within 90 days. Those tires go to the UTS.
Sometimes one or more get injured and can't be repaired, or the owner doesn't want patched tires. The tires, if repairable, end up at UTS along with the others.
Sometimes the owner wants to swap from a summer tire to all season or such. They end up at a UTS.
I think some road hazard warranties and wheel/tire warranties cover replacement of nearly new tires with new, rather than repair. Boom, UTS.
I'm aware of about 200 top name tires that made it to a local shop. They were recalled/reimbursed by the manufacturer for a marking discrepancy. Some distributor in Canada, rather than destroying, put them in a container and sold them to a UTS in the states.
Some UTSs buy up wheels/tires from custom wheel shops. Lots of new car takeoffs.
There's some real junk in used tire piles, but if you're cheap and have a good source for cheap mounting and balancing ($15 to $50 in my AO) it can be an economical way to keep good rubber on cars, especially cars that aren't aggressively driven, high speed, high payload where a failure would be a real danger.
The internet outlets that I named have brought a level of selectivity to used tire shopping. You can get exactly the model/condition you want if you have a little time. Like I said, the set I put on our Toyota this summer were all less than one year old date code, all 90-95% tread.
Outlets like Discount Tire and Sams have more control over where their trash tires end up, but around here all the "new" tire shops give/sell anything good to the used shops to save on the disposal cost that they already charged the customer.
If you buy at a UTS and it won't hold air or is bad when they mount it, you have some recourse. It's hard to look at a used tire and know if its any good, so it's risky to buy from some schmo on craigslist and take their word for it. The legit online outfits are pretty good making you whole if there is an issue.
Used tires are only good in an emergency, or perhaps for use as a full-size spare to replace a donut.
I look at it this way. Even if I buy new tires, by the time I drive them home they're used tires.