Steam How do you actually sell a game via Steam?

I see people selling games in the marketplace forum, and I'm keen to know how its done? I thought once you buy games from steam, its forever attached to your account.

 

What they do is they but a gift or CD key of a non-steam seller, which you then enter in steam. They can sell these onwards.

Games linked to your account, however, cannot.

 

Or they buy a multipack of something and get giftable extra copies.

 

I see. Thanks for the info

 

More information here:

https://support.steampowered.com/kb_article.php?ref=6748-ETSG-5417

I think trading is more common than selling games for cash, not entirely sure. If you want to buy a game you're more susceptible to scammers, but if you're trading both parties get what they want at the same time.