The Ebay Chatter Blog has some questions answered. It appears that Ebay is going to be tracking your computer through cookies and something with Flash rather than tracking your ISP.
They did address the question of third party listing services ... not sure I understand their answer but then that's Ebay.
Q: How will listings submitted through a 3rd party listing service be handled?
A: You should not be affected unless you need to authorize 3rd party access to your eBay account. During that process, you will need to go through identity verification if we do not recognize that computer.