I would only buy lenses used if I could test them out first. If you can, check the AF in live view and then using normal AF. Set up your tripod and take the two shots changing nothing other than AF mode. If you see a difference in image quality, then the lens is not calibrated to your body properly and you will end up paying more than you could have saved by having to have it calibrated. That said, I have bought and sold used gear often and have now problems doing it again.
Canon 7D/350D, Σ17-50/2.8, 18-55IS, 24-105/4 IS, Σ30/1.4, 50/1.8, 55-250IS, 60/2.8, 70-200/4 IS, 85/1.8, 580ex II, 430EX II, 270EX II.