Things to note before answering/reading:
1. I am a PREMIUM ACCOUNT. This is not a F2P issue.
2. The trade server and item servers are up.
3. I don't know if this has anything to do with the issue, but whenever I open up TF2 for the first time after restarting Steam, it opens a Steam box that says "Validating files: Team Fortress 2" and goes through a process of.. I don't know. I guess validating my files..
Now, on to the issue:

This started happening 12/12/12 (I'm serious. No joke). Anyhow, whenever I try to trade someone, it says they're in a trade. Fine by me. I ask them to check. They shift+tab, and they're not in a trade. Weird. Okay. So I ask them to trade me. It tells them I'm in a trade. I shift+tab, says I'm not in a trade, I restart Steam, and my computer. Issue is still there. Alright. I figure the trade server is probably down. I ask people if they have the same issue. They don't. I ask if the trade server is down. Its not. So, I'm confused. I looked on several forums, but I can't find an answer. Please, tell me how to fix this.