@ebm ,
I believe they are using VPN's- Virtual Private Networks in order to shield their IP addresses.
Something I should do also, perhaps they can provide a good VPN company, but I digress...
I know that Audiogon has a protocol that uses IP addresses in order to screen potential misuse. For the longest time, when I used my laptop instead of my desktop, I had my account blocked, until I completed a confirmation of my identity.
This probably makes sense, especially when purchasing equipment, as Audiogon will have a trail to follow in case something goes wrong.
B