I've done used tires many times, but they were usually "Take offs" where someone was trading up tires / wheels on a new car.

I've heard with snow tires, that Blizzaks (and a few other "special" snow tires) only have their maximum ice traction compound for the first 40% of treadware, then they are basically just standard snow tires.

I'd verify any used tires I get don't have any punctures or sidewall damage, have decent treadlife left (At least 6/32"), have pretty even wear across the tire, and be fairly even wear from tire to tire (within 2/32" of each other).