A lot of people (world wide) lately have been complaining about an Error verifying STEAM UserID Ticket, or You have been banned, and Someone using this Steam ID. It seems that the latest round of updates to the Steam platform have caused these problems. Luckily it’s, usually, a quick fix. There are a few actually.
1) Simply restart Steam. This is so simple that a lot of people forget this step.
2) If that doesn’t fix it, and it didn’t for me, then re-login to Steam. Launch Steam, then under settings click the ‘Login as a different user’ button and re-enter your username and password. This fixed it for me, I haven’t had any problems since.
3) Failing that locate your steam folder, usually in the program files directory, and delete the ClientRegistry.blob file and restart Steam and bingo! I can’t testify that this works because the previous solution fixed my problem. If it works, or doesn’t, then leave a comment.
4) If that doesn’t work then hit up the Steam forums and ask there.