Agree with nogoats. Used tires are a crapshoot. If you must buy used, try to buy them from somebody you know so you have a good idea of what they've been subjected to. Wear dot depth is an obvious concern, but you'll also want to punch them with a durometer (at racing air pressures) and look for signs of overheating (feathering or extra wear, usually along an edge). Scuffed sidewalls and / or bent rims could indicate wreck damage.
I don't like to pay more than about 1/4 price for used tires (compared to new tires). Most racers sell the tires they don't like - not the ones they like - although it is always possible that you'll like what they don't. Also consider this: if you pay 1/2 the price of a new tire (or more) then the seller can buy a new tire using your money plus his - but he ends up getting a new tire for what you paid for his used tire (or less). In effect you are helping pay for his advantage over you.