Its not possible to track MAC or HDD as you said , you can be sure they are not tracking. probably your friend was flagged another reason , you should check Stealth Book
the reason why ebay track you is primary your cookies,flash objets in your computer(you must have clean or virgin browser) .,second your Personal information(Address,Phone number etc.),and Third
your IP address.
Thats why many accounts got busted,"People Mistakes"