When a listing goes as unsold, then the listing will automatically be put into Post Auction Negotiation, better known as PAN.
Within PAN, the seller can make an offer to all interested parties, or alternatively, any interested parties can make a single offer to the seller.
PAN is also available when a sale has been disputed and said dispute is resolved neutrally or in the seller's favour.
If an outbound or inbound offer is accepted, then said bidder is declared the winner of the auction.
If the seller chooses, they can also opt to disable PAN or re-list the listing.