Apparently it’s not common knowledge that buying used tires is a bad idea. Usually. Not all the time, but like 99.9% of the time. If you’re replacing your spare, or you need a tire for off-road use, or just to get a car rolling so you can move it: Sure, save your money, buy used. But for actual daily use, generally not a good idea.
Here’s the problem, tires can have all sorts of issues that make them unsafe, or at least give rise to annoyances in use - which are not visible just from looking at them. If you’re not totally clueless you can check simple things like the date code on the tire, the depth of the tread and look for cracking or dry rot. But there are also problems which won’t be apparent until you’ve mounted the tires, like whether the tire is out of round, or has a slipped belt, both of which will make your car ride and handle like crap. Ever bought a used car that pulled like it needed an alignment? Probably just needed new tires. Further these issues may be symptoms of worse problems like a tire that’s about to suffer tread separation.
Another issue is that tires get “used to” the vehicle they’re used on. If the tire was used on a vehicle with bad alignment, they wear accordingly. Put them on another vehicle, and it messes with its handling. Old tires can be bad enough when on the vehicle they’ve spent the last two or three (or more) years on, they’re even worse when you swap them onto a different one.
For the most part shops that sell used will be on the look out for obvious problems like aged tires, or unevenly worn tires from poorly aligned vehicles. And if they’re honest they’ll let you bring a tire back if it turns out to be a piece of shit. Not everybody wants to screw you over for a couple bucks. But on the other hand, some of them just want to clear up some space, get shit tires out of the way, and take some of your money at the same time. Basically getting you to pay them for your taking their garbage. So if you have to buy used, be careful and check the tires yourself, don’t trust that anybody else has.
Now when do used tires make sense? Replacing your ten year old spare tire. Replacing the dry rotted tires on your backyard project vehicle. When one of your tires got damaged but you don’t have money for a new one (or the other three are sufficiently worn that buying a single new one would be pointless). When you’ve got a fun-vehicle that is rarely used, or only used off road. When you want to try out a new tire, but you want to “sample” it first (even though for the reasons above this may not be the best idea). For your daily driver? Usually not a good idea. And even if you do get a good set of used tires for your daily, you’re still getting a used product, which won’t last as long or be as new, and which if it fails you’re usually SOL, and you’re going to be paying the mounting cost again when you replace it, so was the money saved really worth it?
Tires are about the worst part of a vehicle to cut corners on. Whether they’re new or used, there’s no point fixing a vehicle up if you’re going to put tires on that make it ride and handle like crap. I’m a poor-ass dude, and even I won’t settle for old tires, even if it means buying one tire at a time on each successive paycheck, that’s still money better spent and less time wasted than messing around with tires that may have been somebody else’s problem.