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.

I want to buy items from the item bots for metal and keys, how do I buy these?

This is the items page. Scroll down a little and you'll see all the bots.

This is a bot. Click on it to see all the items it has.

Here you choose the items you want to buy. I've searched for "name" and selected a Name Tag
To the left you can filter the itemsif you for example only want to see Engineer or Medic items
To the right you'll see how much you'll have to pay for the items
Hit the green "Next Stage" button to continue

Now you'll see your backpack. Here you can choose if you want to trade your items for the items you selected before. I want to pay with metal and or keys so I don't select anything and hit the "Trade Now!" button.

You're now in the queue to buy the items you selected. You might be in a queue for a minute or so. 

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 the items you wanted. 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.

There are some reasons of why you are experiencing that issue:

1. Your backpack might be outdated, try closing TF2 if you have it open or try restarting Steam.
2. The item you are trying to sell is not on the price list. Scroll down on the item banking page to see it.
3. If the item is uncraftable, make sure to check the chart to see if we accept the not craftable version of it.
4. The item is untradeable.
5. We have hit the stock limit of said item. You can see the limits in the price list.

You will have to wait until someone sells that specific item to the bot,as all of our bots are restocked by users like you.

You can also check the other banking types for your desired item (for example, if you are looking for rare hats or rare stranges, check out item banking)

Then you will have to wait until the bot gains more space by buying or selling (depending on what banking) items from/to the users. Staff will take action if it's needed, but we ask you to refrain from poking them about it every 5 minutes.

This happens when all of the bots from the banking which you are attempting to use are full, as we do not load the inventories of full bots in the general view. You will have to wait until they have more space in their inventory.