I think you're right that people have a visceral reaction to people they perceive as arbitrageurs
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like arbitrageurs in nearly every domain where they exist, they "earn" their profits by providing liquidity and handling risk
I don't think the model where ticket sellers are leaving profits on the table for fear of seeming greedy makes much sense
the model that makes better sense (to me at least) is that they want to set a price where the tickets clear earlier than 5 minutes before the show
(also, like all endeavors where you're setting a price, its hard to precisely predict demand)
the difficulty setting a price, and the preference for early sellout, create an opportunity for scalpers who are willing to buy the risk that they won't be able to selling the tickets for more than they bought them for
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also venues make money off non ticket revenues, so they price the tickets at price that maximizes the profits from ticket revenue + plus other revenue streams (concession, parking, etc),
so they will price it lower than scalpers, who don't have access to non ticket revenue, so they're pricing to maximize just ticket revenue
I don't think there's anything wrong with getting money that you didn't earn (so long as you're not exploiting people, as the bourgeoisie do); I do think there is something ethically questionable about forcing (I use the word 'forcing' to refer to the idea that within the confines of show-going, it is the only available choice) people to pay 3x more for something out of greed.
there's a structural issue here, which is that the world isn't a very fair place and it sucks to be poor. this ticket market situation is not really an example of exploitation though. nobody is being coerced into buying something they don't want, or not getting what they pay for.