I was referring to the blacksmith's quote from randrak, which you altered a bit. And the point I'm trying to make is there is a lot of ambiguous language. The entire shop scene, so nothing is taken out of context:
The Ranger first chooses to go to the magic shop, but the rest of the party remind him that they do not have any money for identification spells. He goes to the blacksmith instead.
At the blacksmith (a half-orc they already know from before) he presents the bundled up armor. The blacksmith studies it, making note that the armor is very damaged (from the battles) and it would lower the price. At the mention of the price being lowered, the ranger asks if he can make a int check to learn how much such an armor would go for, he rolls well. The half-orc notices the magical ring and immediately asks "Are you looking to sell the whole thing?" to which the ranger immediately says yes. The blacksmith offers a price, the ranger isn't too sure since it was lower than expected. The blacksmith argues that with the damage would lower the price and he still needed to make a profit. The ranger still seems to have a hard time agreeing. The blacksmith quickly adds that it's money up-front. The ranger agrees.
The ranger then shows the mithral sword, saying it is from an ancient kingdom. The blacksmith offers a price, but the ranger isn't pleased and the blacksmith does not argue and lets him go.
I'm going to accept your correction that the gauntlets were originally presented as a 'set', but the quote you offer partially undermines even that by noting the different iconography. And, specifically, they appear magical according to the rules set forth by the OP in his game world. This is important for later. Additionally, I'm going to remind you sets themselves can be vague concepts.
The problem here is your think there is certitude on issues that are really quite vague. Let's take a look:
The "whole thing" was the set with the gauntlets and ring, just as the players bundled it, and it seems that the DM already played memory for the party once. The party blew it not once, but twice by forgetting that the gauntlets and ring were a part of the set. Jeeze. I didn't realize the players messed up that badly. The DM did a fine job.
You're losing track of what I am contending. I am not contending the players haven't made mistakes. I am contending the blacksmith scene denied the ranger's player the ability to realize previous mistakes. So, in the context of the blacksmith scene, 'the whole thing' does not immediately equate 'set of full plate armor, gauntlets, and ring'. It means all of something, and contextually, all of what is being offered. The phrase is appropriate in either the scenario of the player offering a set of full plate armor, and in the scenario of the player offering a set of full plate armor with mismatched gauntlets and a fused ring. Thus, it isn't the tip off you seem to think.
As for the issue of the party blowing it twice in a previous scene, I'm not arguing against that. It doesn't matter when we get to the blacksmith scene--the DMs performance in that scene cannot be rated based on the previous scene.
What's unconvincing is your argument that it wasn't all a set and/or that the players were unaware that the gauntlets and ring were a part of that set.
Whether it is part of a 'set' or not (and I have accepted your quote, as noted above, with some caveats) isn't central to my argument. I'm arguing that there is no language specific enough in the blacksmith scene to indicate what you believe was indicated.
It wasn't worth far more than the blacksmith offered. An appraise check would reveal a damaged set of full plate, complete with gauntlets and fused ring which devalues the ring considerably.
So you agree the appraise check should have, at a minimum, given the sensory information that the DM denied the player. I say this because you specify what it should 'reveal', but that was not said to the player. So, is it a DM mistake for not mentioning this? And if it isn't, then why, considering what you've just said, above?
And whether the ring is devalued considerably (and you cannot make that assertion since you don't know how much effort to remove the ring it would take, or whether it works just fine on the gauntlets, but I digress) doesn't even begin to address the overall value here, which is clearly and inarguably increased by the presence of two magic items (obviously magic, as per the DM's campaign rules.) Can you please explain to me how two magic items doesn't enhance the value of what is being offered? The blacksmith immediately recognizes the added value the ring alone (no comment on the gauntlets was given either way) and immediately started trying to purchase the armor, and you're arguing that even with a good appraise roll, the ranger can't figure out the same?