Short answer: This game uses Steam DRM. You *cannot* install and play this game if the Steam activation code is already registered. Your best bet is to buy it new. Wait for the price to drop further if you can't afford it now.
Other options: If you must buy the game used, then be sure to find out if the code is already used. If it is, either don't buy it, or get the seller to include the account login information under which the game is registered as well. In other words, the seller is also selling you the account along with the game. However, please know that this is a breach of the Steam EULA and the account will be suspended if Steam finds out.
If you still must insist on buying it used, then fair-use law says that it is OK to bypass the DRM if you bought the product legally - meaning that you can crack the DRM. However, this method takes some work and PC knowledge. in addition, the websites that provide the cracking methods and tools are usually ran by hackers and pirates - and unsurprisingly, are usually not safe to download from unless you really know what you are doing. Therefore, this is not recommended.