Not sure why we haven't made one of these yet, but, I present to you the TF2Club's Trading Post.
If you're looking for a specific weapon, hat, paint, or trying to get rid of an item you don't need, just post a comment detailing what you have and what you want.
If you want to show another person your backpack, use - www.tf2items.com/
If you want to see a list of all the available items, use - wiki.teamfortress.com/wiki/Mai…
And if you want to check the rarity/ popularity of an item, use - tf2stats.net/items/