How do I buy Team Fortress 2 items outside of the game?

In the last year, Valve implemented a way to buy in-game items in Steam. On this page you can see a link to buy keys on the right side, which links to here

But going to that link gives an error:

One or more of the items requested are not for sale.

How do you buy TF2 items (keys) on Steam and not in TF2?

Solution 1:

While this may not be the only way, this is a way that works:

Go to this site, and scoll down until you see the "Contact us" box on the right hand side of the webpage. Right above it will be another, smaller box with "Supply Crate Keys" on it. Click the link, and you should be able to buy a key (Tested myself)

As to why the wiki site link didn't work, I would guess that the sites link is broken, possibly linking to a old store page

Solution 2:

Mann Co. Online Store

Once you make your first purchase, you get the Mann Co Online Cap (different from the Mann Co. Cap).

Since your post also references keys, you can also try Steam Marketplace. Think of it as a community market for the sale of certain TF2 items, and it's accessible from the main Steam panel.

http://steamcommunity.com/market/