That's a common scam, and why buying used gear from untrusted sellers can end badly.
You really want a massive discount for the risks involved in buying on such services.
With the seller gone, you're left to contact eBay, and to report the transaction.
eBay, Facebook Marketplace, etc., there are sellers on all of those services and others that can be selling what is effectively if not actually lost or stolen gear. They're selling expensive bricks.
In-person sales can end badly too, as devices can be reported stolen after being sold, and you end up with a brick.
This longstanding user tip references iPad and iPhone, though the situation also applies to Watch, and to any recent Mac:
The All Too Common SAD Reality of Buying a used iPad/iPhone - Apple Community