I would say DON'T buy off e-bay for the same reason you don't buy a car-seat off e-bay; they say it's new and not used but do you know what's really happened to it?
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.