I blame 3ed/d20 for this problem. Everything - classes, monsters, NPCs, etc. - all followed the same rules. It empowered players to reverse engineer everything but in the process, DMs lost control of being able to surprise their players. 5E: take the power back!
No.
DMs never lost the power. Ever.
There has always been some form of "Rule 0" that has basically said "What the DM says, goes."
3ed/d20 gave min/maxers, powergamers and whiners ammo to question the DM at every step if he didn't do something per "RAW". How's that?
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.
Basically, should people just stay away from the DMG if they aren't DMing?
I had a player come to me about selling a magic item. He automatically assumed that all he needed to do was make his Investigation roll and then roll for days to find a seller and BAM, his item is sold. I explained to him that's not how it works in my world. "Well the DMG says XYZ", was the response I got and I told him he shouldn't assume anything he reads in the DMG is automatically RAW.
What do you think?
So, by RAW, it's up to you whether selling magic items per the book is available or not.Dungeon Master's Guide p. 128 said:Depending on the style of your campaign and the particular backgrounds and interests of the adventurers, you can make some or all of the following additional activities available as options.
What does this have to do with selling magic items, which is marked as optional?Ask yourself, why should a player assume that a rule which is not marked as optional would not be used in a game?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.