You can think of a market with no walls and no gate. The goods are out where everyone can see them, and anyone who wants them may walk up and make an offer. Nothing is arranged quietly with one buyer beforehand, so the price is simply what people are willing to pay. Both uses of open market keep that picture: one is about a single thing put out for anyone to buy, the other is about a whole trading system with the gates taken away.
One branch
攤開來賣，誰都能出價
兩個意思共用「沒有圍牆的市集」這個畫面，差別只在看的是一筆買賣還是一整套制度。
noun · B2
If something is sold, bought or traded on the open market, it is offered to anyone who wants to buy it, in the normal public way, instead of being sold privately to one chosen buyer.
公開市場（上）