Maybe I don't understand. I thought the OP meant that Audio Advisors asked him for a credit card in order to have a phone conversation. It was not my understanding that Audiogon was asking for a credit card.
To communicate with the dealer, it seems there are a couple of options. I just went to the ad, clicked on the Audio Advisors E-STORE from the ad, then from the E-STORE page I clicked on "contact", and I was given the opportunity to "Message to audioadvisors" through Audiogon. The other option is to simply go to the Audio Advisors website, and from the "About Us" tab, select "Contact Us" and proceed to an on-line contact form, or use the email address provided, or simply call the phone number provided. If the OP made that phone call and was told by Audio Advisors that he needed to provide a CC in order to have a discussion, then that is between the customer (OP) and Audio Advisors, and shouldn't have anything to do with Audiogon. Again, maybe I am missing something.