I want to sell items for metal / keys, how do I turn my items into metal / keys?

This is the items page. Click on the "Sell Items" button to begin.

Here you choose the items you want to sell. I've selected a Muskelmannbraun paint.
To the left you can filter your items if you for example only want to see your Scout or Heavy items. 
To the right you'll see how much you'll get for your items.
Hit the green "Trade Now!" button to continue

You're now in the queue to sell your items. You can see the queue at the top of the screen. I'm currently in a "global queue" which is what you'll be in at the start. Then you'll be assigned to a bot to trade with it. You might be in a queue for a minute or so. 
You can see what the trade needs to look like. 2 reclaimed metal for my Muskelmannbraun paint.

Now this will pop up. Click the green "Go to trade offer"

You'll now get into a trade. Simply accept the trade if everything is alright.
When the offer is alright and has gone through then the trade will be closed.

You'll now see this. You now have your metal. Simply close it by clicking the white X at the top right of the box.

You've now succesfully completed a Items trade! Congratulations!
It's basically the same with all the other trades, but. You can always go and check those tutorials as well.