While I am in the camp of swapping clothes with family and friends, I think this could work very well. I have met enough crazy parents that purchased Air Nike shoes for their non-walking babies to make me question their grip on reality. See
http://www.google.com/search?q=baby+nike+air+jordan&tbm=... for what the stylish babies are wearing.
I would suggest letting parents pick the outfits, since anyone fashion conscious enough to want to use the service will be sensitive to what they get in the mail. I think if you focused on very high price baby stuff you could do well - items priced higher than the typical baby swap stuff.