i wouldn't buy a used timing belt, spend the money and get a brand new oem one, it'll last longer and it'll be alot more durable. you can't prove how many miles are on a belt.
no offense, i can sell you my belt that could say has 80, 000 miles on it and tell you it only has 30,000 miles on it, and chances are you probably won't see the difference
spend it a bit more, and you know its brand new, and in the end you'll save more from having a less chance of that belt snapping on you.
just my thought on it.