Obviously used tires are going to come with previous wear. Depending on where you buy them from, they could have a lot of wear or not.
Especially if you’re buying used tires online, it’s hard to make a good judgment on the condition of the tires. If you can’t see the tires in person before purchasing, then you may not be 100% of what you’re getting.
You simply don’t know what the tires have been through, even if you do buy them in person. You can’t guarantee how many miles they’ve been used for, or what kind of driving conditions they’ve been in so it’s difficult to assess how good of shape their in.
If the tires were stored or used in harsh weather conditions, such as extreme sun or heat, the tire’s internal structure may actually break down over time.
Because you don’t know what kind of wear they’ve been through, there’s no telling how long used tires may last you.