being a mechanic for 25 years, i can honestly say there is no such thing as a good used tire-obviously somebody took it off their car for a reason. it was either wearing out or the car was possibly in an accident. but if you must buy used look at the tread carefully. it should be even the whole way around the tire and across. there should be no
'flat" spots anywhere on the tread. squeeze the tire together before it is mounted and look down in the tread for signs of weather cracking. a tire could look brand new , but it may have been on a car that sat in one spot for 2 or 3 years and that deformity will never come out. make sure the tire is properly balanced and ask the technition if it took any excessive weight to balance it. this could be an issue. if at all possible, i recommend new , name brand tires from a reputable shop and not from billy bobs discount tire imporium. many of these bargain shops sell "seconds" that may also be not fit for the highway. good luck.