Truthfully...
I would not assume anything in the DMG is true for any game world. Sigil is mentioned in the core book - should I assume it exists if I'm playing a Greyhawk game? Should I assume that Holy Avengers exist in a low magic setting, just because its in the book?
My knee jerk reaction was to assume that anything in the DMG is true until proven otherwise. However, when I really thought about it, there's a lot of things that really don't make sense to assume as the default. Given that the GM has absolute control over all NPCs and setting details, including anyone who might buy a magical item, assuming that selling a magic item will be easy as making a few rolls is kind of iffy.
I don't see anything wrong with a player going, "Hey, I want to try and sell this magic item." And likewise, I see no reason why a DM would have to explicitly say that you're doing it differently ahead of time. I don't even have a problem with the player trying to have a discussion with me over a rule. Sometimes, I misremember stuff with everything I need to keep in mind, while players just need to know their character. I do have a problem with a player going "Well, the DMG says XYZ! You have to do it that way!" That's rules lawyering, and is frankly unwelcome.
But a conversation that's something like this - "I want to sell item X." "No, we're not rolling like that. Its different in my world." "Oh, I kinda assumed because of page XX... well, whatever, my character is still trying to sell. How do you want to handle it?"
That's the kind of thing I expect from my table. Player had a minor detail wrong because of something we just hadn't gotten around to talking about yet, realized how things went, and then we went ahead with the game.