This video should scare the crap out of you...for the sake of whoever wound up with that tire.
In it, you see how scam artists take worn-out tires and "recondition" them to look newer and safer so that they can then be sold on the second-hand market.
How do they do this? By cutting in new grooves in and pulling the old rubber out. Then, the shoulders get a once-over as well. Presto: to the uninformed observer, the tires look like they have plenty of life left when, in fact, they're essentially blowout time bombs:
These guys don't care if a buyer lives or dies. They get paid either way.
So do yourself a favor and take care of your tires. And when they're done, go to an authorized local retailer you can trust or hit up a place like . They can help you make an informed choice, and ensure you're only ever buying the real deal.
Because if a used tire seems too good to be true, this demonstrates why that may be the case.