I recently came across the stock market saying,
"Buy on fear, sell on greed."
I'm curious about the grammar/nuance here.
Why is the preposition "on" used instead of "in" or something else?
What kind of nuance does "on" give to the sentence?
Does it have something to do with a certain condition like "on condition" or short form of "Based on"?
Or is there a totally different reason?
"On" has many definitions. One definition is "happening after and usually because of" (Cambridge).
For example: "On arrival, report to reception."
"Buy on fear, sell on greed" means that when you see fear (panic in the stock market, reducing prices), that is when you should buy; when greed drives prices up, that's when you should sell. The preposition is connecting the event to the action.
I will admit it sounds unusual, though. It's because it doesn't explain whose fear or greed it refers to - yours, or other people's? You have to think about it, or have it explained. There are some similar, better-known sayings such as "hope for the best, prepare for the worst" which are said without fully qualifying all the details, but usually those things are tacitly understood.