Private Messaging

From the home page-upper right corner hit the message icon, and you'll see the link at the bottom of the page. Member lookup-message from there.

Audiogon - The High-end Audio Community

Or, find a post he wrote, click on his username, A drop down box will  appear that says 'message user'.

I click on the moniker and then marketplace feedback (the message function is there if allowed by the user).

DeKay

When you click on the member's name, it shows a menu. Click on the "Details".
This opens a new page. On that page click on "Marketplace feedback" button.
That takes you to yet another page. On this page you should see "Send Message" button.

Hope this helps.