By a person bidding or making an offer authorizing the payment if s/he wins/offer is accepted.
eBay has been "experimenting" with requiring pre-authorization of payment for Best Offers for over a year and a half, adding more and more users into the test group required to do so (sellers can opt their listings out of it; buyers cannot opt out once they are assigned). Too many users were making offers and then not paying when it was accepted, and too many of those were scammers who would try to get the seller to text or email them to get their number/email address to send fake payment messages to.
Not sure whether or not they have officially made it permanent yet or it continues "experimentally." They have recently started trying it out on auction bids.
