Garage sales are not just for finding formal clothes or even a cute jeans and shirt outfit. I have also found sports clothes such as sneakers, running t-shirts, shorts, and even sports equipment.
Sometimes sweat can make clothes icky so be careful when buy it. Look at it closely to make sure it’s decently clean. If you are a big time runner, I highly suggest not buying running shoes there. This could seriously hurt your feet.
I am not a serious athlete, so I can find things that suit my needs; but if you are serious, I suggest you spend the money for good quality equipment.