After a contract is completed, the buyer and seller report the price to a central point, and the price is usually written on the blackboard at the time it is reported. The most striking result of the decentralized negotiation is the tendency for quantity exchanged to be too high. The bid is then publicly announced to the market. Sellers' offers are also publicly announced. All bids and offers are written on the blackboard as they are made.
Any buyer is free at any time to accept a standing offer, and any seller can accept a standing bid.
This is a double auction in a sense that bids rise and offers fall at the same time.