To start out, I bought $50 worth of keys. Keys are available through the Mann Co. Store for $2.50 each. This is not the most economical way to purchase them, however. Keys can be bought from resellers for around $1.50 each. This is what I did.
Once I had my keys, I traded them in a trading server for metal. I got a decent amount of refined, and a 2010 Spine Chilling Skull hat, worth 8-10 keys, for 9 keys. This was a huge mistake.
The hat was worth what I paid, there's no question. The problem was, it's rare, and there's next to no market for it. It sat in my inventory for weeks. Finally, I traded it for a Bill's hat (8-10 keys) that was painted Operator's Overalls. This hat is the most commonly traded hat, and it is significantly more liquid than the Spine Chilling Skull.
This didn't get me anywhere, however. In every trade server I went to, I was told that my paint was ugly, and worthless. The best they would offer was 8 keys. I'm still holding out for 9, but I've yet to find a buyer.
Tip #1: If offered a non-liquid item, only accept it at under its spreadsheet value.
For example, let's say you're selling something for 12 keys. Somebody offers you a Bill's hat (worth 8-10 keys) and 2 keys for it. Tell them that since you are looking to move towards a more liquid inventory, you can only accept the Bill's hat as 7 keys, and they need to throw an extra 3 keys into their offer. You'll make bigger profits faster using this method. If they don't accept, who cares? Somebody will come along and offer the metal or keys you were originally asking for.