If you've lived on this earth longer than three years, chances are you've accumulated a lot of stuff: books, movies, games, clothes, baby stuff, old camping crap. You name it, it's probably in your closet. And better yet, there's some one out there that could put it to good use. So instead of piling more junk on top of that unused stuff, trade it in. There are plenty of online opportunities to take advantage of. Here are just a few.