I haven't played TF2 at all, so I'm at a loss at what these 'items' that are for sale will do. Is there some form of persistence with your character now and weapons and tools that change the game and how it plays?
Yeah, pretty much. A lot of this was added after I stopped playing.
They started off with unlockable weapons when they added achievements. If you got a certain number of achievements, then a new weapons would unlock. These were tied to the class updates, so if you got 10 Pyro achievements then it would unlock one of the Pyro weapons.
People didn't like this so much and it caused a lot of achievement maps and stuff so people could just get them over with to unlock the goodies, so Valve changed it so you could get them from achievements OR random "drops". I'm not exactly sure how drops work...if a player actually drops the weapon for you to pick up or you just get a random chance for it to unlock every time you kill someone. There was also a simple matter of unlocks happening the more time you played so people made idle servers that they would sit in and let stuff unlock.
The problem with random drops is you never knew what you were going to get and you might end up with duplicates. They eventually added "crafting" so you can combine duplicates to make the item you do want.
And now you can just buy the things. And at some point they added funny hats in addition to weapons. TF2 is a clusterfuck of ideas. The game is basically a mess.