"Jeder an der Börse dachte das."
Why "AN der Börse"? Are there other offices or institutions that needs "an" as preposition when an action is taking place there?
It's a collocation you have to remember. It's also used with 'Bushaltestelle' (fermata d'autobus). Another notable example of a prepositional collocation that confuses many learners: 'at court' (judicial) = 'vor Gericht', 'at court' (royal) = 'bei Hofe'. A little helper to remember it: we usually use 'an' for locations at rivers, lakes, the sea, ... The stock market is a place where your money flows away ;-)