thredUP. This site is a used clothes swapping site for kids. What you do is purchase a box of clothes (usually between 10-20 pieces per box) for $5+ shipping and when your ready, you can list a box of clothes for someone else. They will ship you the boxes to put your clothes in so you don't have to worry about that and you don't have to pay to ship it either.
They have all sizes for kids, boys and girls. I just signed up for the site a few days ago and they gave me a code for a FREE box. So click here and maybe you too might get a free box. What I heard is you must read the descriptions for each box. Some descriptions include pictures or you can request for pictures so you can see exactly what your getting. The more information about whats in the box, the better.