A Usenet abbreviation, still occasionally used on various for-sale boards. Literally means For Sale. Usually used in the subject line along with the item that the user is selling, as in "FS: Hamster cage & supplies." The message body then contains a brief description of the item, an asking price, and contact information.
With the advent of on-line auction services, many opportunistic posters will instead use "FA" (For Auction) to promote their wares. In that case, the message body usually consists of little more than a link to that auction's page on its respective site.