Li Shenron
Legend
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?
The closest thing to a default in 5e is Basic, certainly not anymore PHB+DMG+MM.
Even then, I'd say definitely not everything in Basic is required.
What you should do, is make sure to tell your players what is the default of your games. Everything else doesn't exist. The DMG in particular is explicitly a collection of variant rules and modules.