It is against the HF rules specifically:
A Member of the Trade may not--via the forums, Private Messaging, or Head-Fi's e-mail relay function--volunteer any information about a specific product that he is selling or making, in response to a general request for information about a type of product or in any other discussion in the Main Forums.
No advertising by Members of the Trade that is not pre-approved by the Head-Fi administrators is allowed in the member forums. Some sponsors have Branded Sponsor Forums in which they will be allowed to advertise their wares. There will also be designated areas and forums where sponsors and non-sponsors alike will be able to advertise their wares. Head-Fi is most definately NOT anti-trade. In fact, trade members are a welcome and vital part of the Head-Fi community. However, Head-Fi's Main Forums, its Private Messaging System, or its e-mail relay function are not to be used for unsolicited self promotion and advertising of audio products or services.
http://hfimage.head-fi.org/rules.html