This is a really hostile way to approach things, and for a lot of us, that degree of "pay attention" is not physically compatible with how our brains work. Even medicated, I simply can't keep track of things consistently; stuff just sorta slips my mind. Even really obvious stuff. Even stuff I care about.
Luckily for me, I play with adults who are more interested in having fun than showing off how not-disabled they are, and everything works out fine, because if something's obviously inconsistent with what's immediately obvious to my character, the GM or another player will usually prompt me about it.
So, I agree that there's a reasonable approach to this sort of thing, and as a DM I'll often remind players of important things they may have forgotten. Part of it is taking into account the players themselves. For example, I don't expect the same things from my 12-year-old daughter as I do somebody who has been playing for 20 years.
Then it's a question as to whether it would be obvious that the character forgot about the gauntlets vs you forgetting about them. And the "you" in this case is all of the players.
So, first off they have decided that the armor, at the very least, is not magical since they are just looking to sell it. Yes, he tried to get things identified first, but never noted that he would be keeping the gauntlets with the ring attached.
Next, the DM did give clues, whether you think they were sufficient or not is a matter of opinion. They were an attempt to remind them that there was something important to consider, and they didn't pick up on it.
There's no real description of how they presented the armor. It is stated that it's bundled together with rope, so my assumption would be that it's still in the same bundle until shown to the smith. Having worked in retail for a great many years, most people have a tendency to just dump something and let you look at it. So I think this is all very reasonable. I probably would have fleshed out the interaction a bit more, and if they character had just dumped it on the counter, that would lead to different situations than if the player had started to remove parts of the armor to highlight the quality, craftsmanship, etc.
Which brings us to the question as to whether they remember the gauntlets, or notice them on the counter. As I've said, in my game this would be a passive Perception check, because there is no stated suspicion, nor an active search occurring. You're just checking to see if they happen to notice something.
If they were actively showing the pieces to the smith, then I probably wouldn't have even bothered with a check. That's a point where I would mention the gauntlets. Probably something like, "these are fairly beat up already, I'll need to do some repairs, and these gauntlets aren't even of the same craftsmanship, and will certainly lower the value."
I'd argue that if the smith thought he was onto something, he wouldn't have asked anything. He'd offer a price and see if he can make the deal. Having him verify whether he was selling "all of it?" is a pretty good hint in my book. It makes the player think about it, it's a question that doesn't entirely make sense, but it doesn't blatantly tip the hand of the NPC like asking "including the gauntlets?"
This is the point where I probably would have made a passive Perception check against the Deception or Persuasion check of the smith.
My daughter has a lot of difficulty focusing and remembering things as well. Which is one of the reasons I wanted her playing D&D with me. It's a great learning tool for her. Which is also why I make sure she's taking notes, etc. The group of players makes a difference in how I handle a specific scenario, including this one. But to me it was an opportunity to have an interaction in a way that doesn't always present itself, and for the group to have a little fun, even if it's at their expense.
Ilbranteloth