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."
What was different about certain editions before this one, is there were more explicitly set out rules for more situations. Nevertheless, you always had the power to veto them.
It's just that some DMs, for some reason, didn't feel they could veto a more explicitly set out rule. Well, that's on the DM; "rule 0" was still there for you to invoke.
Yeah, it forced one to have a good reason to veto/change something, but that's a feature.
5e seems to be a bit more lax on some of these situations, perhaps "empowering" the DM to make their own calls, but that seems silly to me since the DM has always had the power to make calls. Always.
It was completely logical for a player to consult an official book regarding the selling of magical items. The DM is fine to say "Not in my world." That last part has been true since the red box.
One would hope that, through interaction with the DM's world, the player would have gotten the sense that magical items can't be sold...if he/she didn't, then the DMG is a logical place to go for the info.