I started selling spare weapons I had and buying shitty hats. Then I'd wear them around competitive servers and sell them. I made a decent bit of profit from that, which I invested in Strange-quality weapons (There are different ranks). I made most of my work in Stranges, though expanding into item-value trades was a good idea. Eventually, I got to the point where I was trading 5-10 keys worth of value per trade (to put that into normal-people terms, that's roughly 7.50 to 15$). I had bought a full-coloured collection of my favourite hat, as well as an unusual, tier version of it with my preferred effect. I ended up getting bored, selling all of my items and shit, then droppping the money into paypal.
Scratch that, I just checked my backpack and I still have a load of stuff left. Fuck. This is getting my trading compulsion up. Bob, wanna buy a ton of hats? I'll give you a good deal.