No, you don't go to the forums at all. You follow the link I posted above and in the top part click the tab "ask a question"
I'd strongly suggest you don't use their account. More than likely, if the seller is shady, they will contact Steam support, say their account was stolen. Steam support will then return his account to him, and potentially disable your legit account in the process.Finally! I can play RO! The person I bought the game off of on ebay gave me their Steam account. See you on the battlefield!