For many years I used to use the 'Marketplace feedback' button to send an IM. Seems it is often not working for me now though.
But, I also just found that if you click on the username, NOT the icon on the left of it, but the spelled out username on the right of the icon, then you will get the option to 'Message User'. I just sent a message to a good friend and it did not require me to register a credit card.