For the most part "used" tires are something I stay away from. That's not to say that you can't get a great deal on some "good" used tires, but you better know what to look for. Here's some tips. Have a look at the tread depth (use your thumb nail and the side of a dime as a gauge) make sure they're close on all tires in the set. If the tires were never been rotated - one will have a lot less meat on it. Lots of tread is good, but look carefully for the "wear bars" in between the tread. You want to measure from the top of the wear bars to the top of the tread, not from the lower surface (face) of the tire. To the untrained eye a tire that is worn down to the wear bars will still have some tread left and be mistaken for "good for one season". Wrong, the tire is finished and should not be used at all. In the rain that tire will hydroplane you off the road and into an accident. Have a good look at the sidewalls for cuts, gouges, scratches, big lumps and cracks, none of those are good. If the raised lettering on the side wall has been worn off, that's not good either. All of those are clues to the unseen sidewall damage that causes tires to fail. Uneven tread wear (dips and ripples in different spots of the tread) can also indicate cord or belt damage and /or separation (again not good). Next have a look at the lip area that contacts the rim. If it's well chewed up, the tire has been on and off multiple times and is probably stretched - not good. Have an extra good look inside the tire with a flashlight, is it smooth and clean with no signs of blemishes and cracks (good), if you find a small chunk of rubber stuck (that you can't wipe off) under the tread area, it's the end of a plug that was used to repair a flat. One or two in a set of four is normal, but if you find a bunch of them, or worse yet, what looks like those big round (or square) patches from yesteryear .. walk away. If you see those patches on the inside of a sidewall .. run away - the tires aren't worth a penny. Barring all that, if the tires are for a utility trailer or for a vehicle that's only going to live for another year or two I'd buy used, but remember - If you aren't the one who put the mileage on a set of tires in the first place - you can never be sure of what kind of abuse and damage the tires have suffered and, more importantly, when they're going to fail. The consequences of a blowout on a steering tire at highway speeds are usually devastating.