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.
可以想成一個沒有圍牆、也沒有門的市集。東西就擺在外面，誰都看得到。想要的人自己走過來出價，沒有人事先在後面談好。價格就是大家願意付的數字。open market 的兩個意思都還帶著這個畫面：一個講「一樣東西攤出來給所有人買」，一個講「整套制度把關卡拆掉」。