I consider tires a safety item, and thus never recommend buying used tires.
As for treadwear difference, most manuals for AWD cars will state something along the lines of keeping the rolling circumference the same on all 4 tires (which means tire rotation is actually something one should keep up on with an AWD vehicle). However, a good guideline is 2/32's max difference. Any more than that, and I wouldn't run them.