You can display unwanted items and your friends can bid on them. If no one bids on your items for about a week or so, your items will get sent back to you.
I think it's a neat concept, but none of my friends, even back when I played a lot, used it much. The few that did put common items up with ridiculous prices. I only made one deal on there myself - a crappy shirt for about 100 bells that someone took. Everything else was sent back to me.