So can I. The ring of X-ray vision is one of them.
But I don't recall the referee mentioning any of them (eg that the blacksmith had such a ring, or that the blacksmith took the armour into another room to examine it, etc)
But the ranger was - as far as I can tell - aware that the gauntlets and ring were special, and not part of the armour set (as the players, and their PCs, were conceiving of it). So this is not a case of missing something in plain sight. It's a case of the ranger not having seen the armour that the blacksmith inspected.
Which takes us back to the question of how the blacksmith got to see what the ranger did not. As you say, this can happen - but the GM hasn't told us how it did happen, and my feeling from the two actual play reports is that the GM didn't indicate to the players, either, that the blacksmith was inspecting the armour in some secret manner. When (as per the OP) the GM tells us that "The armorsmith checked it over and when he noticed the ring he casually asked if the whole thing was for sale", I don't think he is implying that the checking it over happened in some covert fashion.
This is why I find the situation puzzling. The ranger cared about the gauntlets and ring, and - as far as I can tell - could see the gauntlets and ring, yet was deemed to have sold them out of absentmindedness. As I posted upthread, I think it's quite different from (for instance) the ranger selling the whole bundle as a job lot and forgetting what has been bundled into it.
Based on the descriptions given prior, the ranger didn't have any idea that the ring and gauntlets were even there. He was not present when the barbarian and monk were inspecting them initially, and was not present when the barbarian decided to bundle them up and take them as a set. Nor did the barbarian or monk ever say anything to the ranger regarding them.
What I see happening here, and it's not likely to change at this point, are several distinct points of view:
The first perspective answers the original question "Was I wrong" with a "yes." This position is largely predicated on the belief that the DM deceiving the players in any way, and specifically by not asking about the gauntlets, is wrong. This is a valid perspective, and the right perspective for a number of players.
The second perspective is that it's not the job of the DM to remind the players of things they should be keeping track of. If they didn't remember it, the character didn't remember it, and deal with it. In this case, the answer to the original question is "No"
A third perspective allows for DM deception, but it must be addressed in a way that works in the game. That is, the mechanics of the game must provide a possibility for the players to discover the deception (on the part of the DM, and by the NPC).
This is what I think. I don't think the DM was wrong in taking advantage of the fact that the player wasn't paying attention/forgot, etc. What I do think, is that while the DM was not wrong in the scenario at hand, he could have run the scenario better.
The barbarian described bundling up the suit of armor with a rope. I envision him putting the rope through portions of the armor he could, and other pieces being in the middle of the bundle. Probably within the chest/backplate. This would include the gauntlets. A portion of the gauntlets may or may not be visible with a casual glance.
The ranger, who at this point had no direct knowledge of the gauntlets and ring as described in the extended version of the encounter, was given the bundle as is and asked to go sell it. The failure in memory was on the part of the barbarian and monk. Could he have allowed a check here to remember them? Possibly. Not sure if I would though. Depends on the conversation and actions going on at the time. The ranger has no reason to single out the gauntlets or any other part of the suit of armor as unique or special, nor any reason to suspect the smith as being deceptive.
He places the bundle on the counter. The smith may not even need to unwrap anything, just kind of poke around to see what's there. He's seen plenty of suits of armor before. He'd want to make sure the major pieces are present, but he's noted that it's been damaged, so a missing gauntlet or something small probably doesn't matter anyway. If it's really unique he might start to untie it to take a closer look at some of the pieces. The ranger, again not having any real reason to suspect potential foul play, is waiting patiently, but not overly attentively.
The smith notices the ring on the gauntlet as he's moving things around. He then stops pulling things apart, in an attempt to hide the ring, or at least prevent it from coming into view of the ranger. They discuss the price, and he asks if it's for all of the bundle. The ranger says it is.
I would have recommended at least a slight of hand and/or deception check against the ranger's passive perception. This is because the ranger still has no reason to suspect anything is amiss. He didn't know about the gauntlets or the ring and they aren't visible. Depending on the results of that check, the ranger may or may not have detected anything. The longer the interaction and conversation continued, the more checks I would allow. Actually, the way I handle failed skill checks in my campaign is it usually represents not an outright failure, but just not success yet. In which case as the conversation continued, I would let the ranger know that there's just a nagging feeling that the smith isn't totally on the level. How he proceeds with that information is up to him.
A better description on the part of the DM, along with a skill check or two would have been a better approach to resolving the scene. I don't think the DM should have mentioned the gauntlets and ring, nor do I think the smith would have. He's hoping that the ranger hasn't seen, doesn't know what he might have, has forgotten about them, or just doesn't care. Since asking specifically about them could result in his not being able to swindle the ranger in three of those four choices, I think he'd keep that information to himself.
I don't think the ranger "didn't intend to sell the gauntlets or ring" because I don't think the ranger (or the player) knew about them at all. There's nothing in the description of the actions that led to it that would indicate he did. I do believe that the other players didn't intend to sell them, but they needed to mention that when they gave the package to the ranger.
Overall, the situation that presented itself to the DM was a great opportunity for an interesting encounter. Even though they lost the gauntlets (for now), they've also learned something about the smith. Perhaps it also leads to additional adventuring opportunities, as further investigation shows that the smith is part of a fencing ring that steals valuable items and smuggles them out of the city to other locations.
I land in the final category because it offers interesting game-play, and endless possibilities. If the DM is just going to remind them that the gauntlets are special, and it's just a conversation about the price of the armor without them, then it's beginning to sound like a scene that doesn't need to exist at all. I'm not saying that the role-playing aspect isn't worthwhile, nor that there aren't other possibilities, but overall it's just a question of how much he can get for the armor, and then the decision as to whether they feel that's enough or not.
So my answer remains, no, the DM was not wrong, but could have done better.
Ilbranteloth