Did you see ThredUp on the news? I saw it on the local FOX news (AZ) shortly after seeing an ad online. I saw the online ad and thought "meh, probably a catch or scam" so I didn't investigate. Then, when I saw the news story I thought "wow, this is pretty cool" Basically when you sign up they send 10 boxes so you can fill the boxes with your child's old clothes to "trade up". Then, list them on ThredUp just like eBay. Every time you list a box you can pick a box to receive! (you pay $5 fee + 10.95 shipping)
ThredUp sends you a prepaid shipping label when someone picks your box and you can have USPS pick it up
The only thing you ever pay is $14.95 for the box you choose to be shipped to you. The boxes usually have about 15 items. That's $1 an item average. Definitely worth checking out!
There's a video of the CEO on Fox Business below (I couldn't find the local one)