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."