Those darn crates are like crack to me. I cannot leave them unopened, as then Obsessive Compulsive behaviours kick in. I thought then, ok I'll buy a bunch of keys so I don't have to keep buying them whenever a crate drops, and then I found I would be trading for crates all the time just to open then, a week later I have spent much more than I am ashamed to admit. Much much more than I would have paid for any game outright.
It comes only second to the amount I've spent on World of Warcraft expansions and monthly fees. Though by the amount I am spending per week on keys, the WoW addiction is cheaper, so I'm back playing WoW, go figure. I actually wish WoW would introduce more personal cosmetic changes for money.
As for trading itself, I have mostly been using TF2items to trade, but also trading a lot of stuff in-game as per requests.
I also have an unusual vintage tyrolean with flames, which looks pretty awesome.
As with TF2, whenever they introduce anything new, it gets hoarded over for a week or two until everyone has the new stuff, and then it dies down. I'd rather they keep adding fresh stuff to the game and keep it alive, rather than just let it stagnate and die.
However the game is getting to a point where adding new content is just going to over-saturate the existing content. It's nearly at that point now with the huge selection of weapons available, and hats adding to that mix.