There is no problem with that.
Especially in this scenario where something like the following could very likely be what played out:
Player: I show the blacksmith the armor and tell him I'd like to sell it.
DM: You present the blacksmith the armor, the gauntlets, and the ring stuck on them. He starts looking them over to assess what to offer you and asks "All of it?"
Player: I said the armor, and I meant the armor. The gauntlets and ring were never presented to the blacksmith because I didn't say I did that.
DM: Oh, sorry... I thought since you guys put all that stuff into one bundle that if you were selling anything in the bundle it would be everything in the bundle.
Player: ...what? That makes no sense. Would you think I was selling my entire backpack and all of it's contents if I told you I wanted to sell one of the items I had previously put in the backpack?
Which is exactly my point. You've reminded them that there is something unusual about the armor, which if they were paying attention wouldn't need reminding. That reminder changed the course of the interaction simply because you mentioned them.
Your concept of selling the backpack (including in your later post of selling the rope) makes no sense. An item you put into your backpack is something entirely different than something that was found, and described, as one item with unique features, and then "bundled up as a set" by one of the players.
So, you find a well-used backpack with several repairs and patches. Two of the patches happen to be special pouches magically attached to the backpack, large enough to store a scroll or potion. The patches can be removed and attached to any pack, cloak, or similar item made of fabric.
The players make a note of it, and plan to investigate it later. But one of the PCs, not paying attention, doesn't really consider that there is anything more than just a couple of patches on a beat up backpack.
As soon as you mention the patches in the interaction, you are calling out attention to them, virtually ensuring that they won't accidentally sell them.
Again, the PCs really have no idea that there was anything more to the patches, or the gauntlets and the ring, since they never investigated them further. They might suspect something was different, but this is more meta-gaming based on the fact that they were described differently. If the DM describes many treasures or items in this amount of detail, then the players won't know without further effort which are just different because the original owner found a pair of gauntlets they liked and added them to their existing suit, and wanted others to think the gauntlets were magical by attaching a ring to them.
To me, it's no different than the players missing a secret door with a treasure behind it. You give them the clues, you let them determine what to do with it. If they decide not to investigate it, you aren't obligated to remind them of it if they return to the room later.
Life is extremely complex. People forget things all the time. How many of us have put something on the kitchen counter or by the front door so we don't forget to take it with us, only to remember an hour after we left that we were supposed to bring it with us? This is a perfectly reasonable thing to expect and allow in the game.
There
are times when I think it's appropriate to provide reminders, or a chance by die roll. Most of the time this relates to trying to remember things that you should know. Perhaps it's clues that you gave them in a session 3 months ago, and they can't find their notes (or didn't take any). They are specifically discussing things relating to those clues, and they are trying to remember everything they know. Then it makes sense, they can make a check. But they are specifically stating that they are trying to remember what's going on.
In this case, though, all of the players had the opportunity to remember that there was something different about it. Based on the OP, they decided to investigate them as soon as he got back. Any one of them could have said, "go sell the armor, we'll take a look at the gauntlets and ring after you get back" before the ranger headed off to the smith.
During the transaction, I'd be listening to whether the player said anything at all that would indicate that they remembered the gauntlets and ring. For 90% of tables, this would include what other players are saying, since that would also change what the PC is saying.
I don't see any way that the DM could mention the gauntlets and ring in this situation without entirely removing the possibility that the shifty smith can't pull on over on them. And while the DM isn't playing against the PCs, at this point in time the NPC is.
Even having the smith point out that the gauntlets are different and attempt to reduce the price of the armor because they don't quite match would have reminded them and would have ended the possibility of the smith getting the deal.
The players should learn from it - not that everybody is a thief, but that some people might be. And they need to make sure that they are clear about their intentions. The DM described them in detail, and the PCs thought something else was more important at the time (the sword), and just said, "keep the set together, we'll look at it later." It appears that it all happened in the same session, so it's not like there was a long time between the two events where they would easily forget.
Better communication between the players (characters), more attention to detail, less looking at their phone, better descriptions of intentions and actions to the DM - all things that are valuable to playing the game and things the players can (should) learn from this.
The DMs job is to present the scenario, and referee the outcome. They aren't there to prevent you from making mistakes or poor decisions. He didn't give them the suit of armor with the intention of taking away the items. He just saw an opportunity for an interesting encounter. The actions of the players and characters are intertwined and I think that's a perfectly reasonable interpretation based on everything that had been described before.
The only thing that isn't clear, although I think we know the answer, is if the ranger had specifically singled out the gauntlet and the ring as being important ahead of time. If they had been the player that was asking questions about them - that is their character seemed to have a particuar interest in them - then I probably would have allowed a Perception check to notice that the gauntlets were present when he opened the bundle for the smith. Perhaps an opposed check against the smith's Deception or Persuasion.
If they had thrown the suit in a sack with lots of other things and had to fish it out in pieces, that would also be different. But to bundle it all up as a set, I see that as a single unit from that point forward unless they state otherwise. Unless they stated something different, "I give the armor to the ranger" is "I take the bundle of armor from my pack and give it to the ranger." If that's not what you meant, then be more specific next time.
Ilbranteloth