Check your account settings at Paypal, via the gear-shaped icon on your Paypal home page which is just to the left of the "log out" button. You may find, as I did, that you now have a merchant ID. (I do NOT, btw, have a business account with either Paypal or Audiogon).
If a merchant ID is not indicated under your Paypal account settings, at the Audiogon homepage click "Dashboard" on the drop-down menu that appears under your screen name, when you point to your screen name at the upper right. Then click "Account" at the left, and find and click a link that will appear which says "Don’t Have A Merchant ID?" You’ll find what appear to me to be fairly simple and reasonable instructions as to how to proceed.
Also, just above that link is a box in which you can enter the merchant ID into your Audiogon account settings. After doing that you would click "Update Account" near the bottom of that page.