The reason for not buying carseats on Ebay is that you don't know if they've been in an accident and therefore there are safety concerns. I hardly think shoes fall under the same criteria. Not all of the shoes are new. Most say "like new" and it's quite easy to tell if they've been worn or not. What do you think the risks of buying second-hand shoes would be? I doubt my child is going to catch some horrible disease from a pair of shoes.
A podiatrist told me second hand shoe problems are due to the fact that all people walk differently, ie if a child walks on the side of their feet, then by your child wearing those same shoes, it may encourage the foot to naturally fall into that pattern of walking.