It is frowned upon to post for sale items in the forums. If you are actually trying to sell an item it needs to be posted as an actual for sale item in the classifieds section...not the forums.
Actually it's not frowned upon, it's totally against the site's rules.
Here's the relevant section from the Guidelines for Post Content:
"All goods transactions are to occur in the Classifieds section of the website. No one may advertise items or services available for sale or trade outside of the Classifieds section; this includes advertising a Classifieds item on the forums. No transactions dealing with copyrighted software will be allowed at all; please see this announcement for details."