B.H.
there is a place in brooklyn- fort green to be precise- that's all used baby/kids maternity clothe: http://stillhipbrooklyn.com/ plus the brooklyn flea market is near there- old and new. i did a google search for "used kids clothing brooklyn" and there were a few spots, you could try that for whatever part of nyc is closest. keep us posted! i'd like to know more too!!