I started playing Team Fortress 2 not so long ago and overall I like the game a lot, the only thing I am missing is some items and weapons. Can you tell me where I can buy them?

You can find a large selection of items for TF2 at Marketplace Tradeoffers - https://tradeoffers.tf/orders/440. On this marketplace you can always find fresh and interesting items that you won't find elsewhere. For example, I recently bought a severed head of Max (a particularly dangerous killer) there, which I really wanted but couldn't get in the game no matter how hard I tried.