When you buy old tires there is no way you can know what conditions they were stored in during that time. Sometimes 3 of the 4 tires will be OK but they toss one really old one in hoping no one will notice. After all, how many people will actually read the date codes on the new tires they just bought? Like food stores, tire places are supposed to work on FIFO (First one In-First one Out) to rotate their stock and always have the oldest stock rotated out. Sometimes they find an old one and have to pass it off onto someone. One of the dirty little secrets of the tire industry. Don't be that someone. Also, when balancing a tire, if they have to use excessive weight and multiple times to get it balanced (and it isn't a bent rim) that tire is defective and should not be used but many tire shops will just keep throwing weights onto it and hope the owner doesn't know any better.