I only have 1 DS and when he was smaller, we would always buy him a new pair of shoes for the warmer months and a pair of winter shoes for the colder. And rainboots. Now that he's older, he has more pairs of shoes (like dress shoes for special occasions and crocs and keens and snow boots) and it gets pricey.
I always thought -- everything used except for underwear and shoes, but I've been lucky to find some gently used shoes that were in really good shape and I won't hesitate to get more in the future. But I'll always buy him new shoes as well if we can afford them.
I guess I reevaluated my position on used shoes, hopefully the same won't go for underwear