I end up buying used tires all the time. I need some tires for my Jeep and was wanting 31x10.50R15 mud terrains but new it was going to cost about $600 out the door so I started looking on **** and a jeep forum and found a guy just 45 minutes away that was selling a set of BFGoodrich Dueler M/T's for $200 a set with 11K miles. Those tires are very nice (low noise, etc, unlike regular M/T's) and were $150-175 new each. So I picked them up, paid my local shop $6/tire for mount/balance and that was it, $224 and they are awesome!
I have ordered from tirerack and discounttire before too for my sports car tires and both are great. Tire rack is a little cheaper, but you might have to pay $100 in shipping, but discount tire usually is a litlte more expensive to make up for their free shipping offers, so just compare them.