I have not seen much about this issue with second hand PC games that need activation via Steam but I know I can't be the only person that's had this problem.
I bought a second hand genuine copy of CoD Modern Warfare 2 on ebay but when I try to install the game it connects to Steam but they won't accept the CD key reg code that's on the manual and I get a duplicate CD key error, this is because the game has been activated on another Steam account by a previous owner and Steam now assumes I am trying to activate a copy/pirated game.
I contacted Steam and sent pictures of the manual with the CD key printed on it and a screen shot of the paypal transaction when I paid for the game but they are adamant that they will not reset the CD key to my Steam account, So, I am left with a completely useless game that can't be activated and therefore will not install and can't be played, great
Now I understand why so many people hate Infinity Ward and Activision and it's also a good way of killing the second hand game market.
The moral is, when buying used games make sure they don't need Steam activation, I have also sent the suggestion to ebay that sellers listing games must state if the game needs Steam activation this is a catch 22 situation because if a used game needs Steam activation it is essentially useless and worthless and not worth listing as it is not fit for purpose and you should get your money back if the seller does not declare the game needs Steam activation
I'm sure ebay will agree with my suggestion and sort out this problem straight away