Then I guess the question becomes this. Should you, as a player, go and read up on the rules in the DMG and assume they are a part of the game?
I don't think it was my fault that I didn't discuss how selling magic items went because I feel like the player shouldn't have gone and read the DMG and assumed that's the way it was done in my game.
I dunno. That's not a relationship I'm accustomed to, as players in any of my games also run other games and buy the books to do so. I guess if you're in a group with a single permanent DM, made up of people who don't want to run their own games, you could all agree that the DMG is off-limits as a sort of social contract, but that would be a thing your group has decided.
Communication is never a bad thing, and this sounds a lot like a bit of communication could have negated the entire issue. Talk to your players and make sure you're all on the same page and have the same expectations of the game, and it should work out.