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.