eBay don’t advertise how it’s determined that a seller is a “new” seller. There’s no magic formula, because sellers don’t only sell one type of item, at a particular price point, in a particular way, at a particular volume, with a particular transaction history / dispute history / dispute resolution history.

As said, sellers will find themselves back in the new seller designation if their selling pattern changes down to a noticeable trickle or cessation of a particular duration. There are holds if a seller’s selling pattern changes from low-value to high-value items; and so on.

All of this is an anti-fraud measure, mitigating the risk of hijacked dormant accounts and the advent of newly created accounts trying to swoop in for a quick profit before disappearing.