ThreadUP is a national clothing swap, basic membership is free!, where you can pick out a box of clothes and all you pay is the shipping. You then list a box of used clothes of your own to complete the swap. You don't pay to ship your box out to whoever picks yours.
How it works (from their website):
Select a Box of New Clothes
The average American family spends more than $1,000 each year on children's clothing. We've made it easier than ever to save time and money finding new gently worn clothes for your child that will look and fit great. Swap with friends or exchange within our national network.
Browse kids clothing boxes listed by parents you know
At thredUP we are committed to facilitating exchanges with friends - whether they live across town or across America. We know clothing swaps are hard to organize, so we've gone and done it for you! It's easy to pick boxes of clothing uploaded by friends you've added to your thredUP Inner Circle. Simply navigate the gently-worn clothing listings the way you would on eBay, iTunes or Amazon.com. Sort by gender, size, brand, season and type of item. Find a box you like, it's yours.
Find gently worn clothing boxes listed by our national network of parents
If you don't find a box of gently worn clothes you love from within your Inner Circle, search the listings of 1,000's of other kids clothing boxes. You can sort by gender, size, brand, season and type of kids clothing item. Find baby clothing, toddler clothing, or pre-teen clothing.
As a Pro Member, we even let you view detailed box contents and the credibility of the sender on things like quality, style and reliability. Pick with confidence based on the combined ratings of peer-reviewed members. Our list of used-clothing boxes are always changing as members regularly swap out outgrown clothes for new clothes, so if you find something you like it, you better grab it!