As long as they seem to fit well it's probably fine, we have used both new and pre-used. But shoes form a bit after the users feet, and if they're used a lot (as shoes are as opposed to rotated clothes) they might be less comfortable and supportive than one would want, and kids depending on age aren't great at realizing they could do better.

If we get used shoes we

At least that's my experience and interpretation of my kids, but YMMV.