How to install and play a retail game when Steam is required to play? [duplicate]

It sounds like you bought unpacked files for the game, without a key. Unfortunately, this is likely a scam. Additionally, trying to move extracted files and then play the game on a new system frequently results in issues (in my experience, at least). I'm hoping this isn't the case! Try and ask the seller for a CD key or Steam key. If they refuse to provide one, it's likely sham-town

You'll have to buy the game from Steam or a retailer. If you want to play a Steam version of a game, you will have to activate the game on your account. The files themselves won't run the game in many cases due to requiring the Steam client to be running.

Keep in mind that if the person already activated the key and just sent you the files, it won't work, and you will need a new key that hasn't been activated before.

Sorry, but you may be in for a problem. Contact PayPal immediately. I have been victim of this before.

Steam will not allow you to transfer rights for a game, so people buy the game and activate it with their own Steam account and it is permanently theirs whether they keep the disks or not. The activation code will not work for any other account, it gets permanently tied to their own. Calling Steam will not get it fixed. They don't care.

I paid $30 at a store for Half-Life only to find it won't work and that I could have bought it through Steam for only $10.

The only thing that the disk may be useful for is if you have a poor internet speed or cap which makes downloading the game impossible. Yet you would still need to download some substantial decryption keys to make the game playable and you would obviously require the Steam console itself.

Sorry to hear that you got scammed. Best of luck.

As MonkeyZeus pointed out in a comment, once the key is activated on Steam, it's permanently tied to the account. If this is the case, you could technically play the game if the seller gave you the login details to his account. It's unlikely to be the only game on his account, so he probably wouldn't want to do that, plus you'd have to log into a different account for that one game (accounts can't be merged). Additionally, this violates the Steam Subscriber Agreement without explicit permission from Valve (emphasis mine):

You may not sell or charge others for the right to use your Account, or otherwise transfer your Account, nor may you sell, charge others for the right to use, or transfer any Subscriptions other than if and as expressly permitted by this Agreement (including any Subscription Terms or Rules of Use) or as otherwise specifically permitted by Valve.

Also, it's a risky thing to try, since the seller can then claim his account was hacked and get Valve to restore his access, and there's not much you can do about it.