Because the best I can tell from the description, the barbarian handed the ranger a bundle of armor, which he simply took with him. He didn't examine it, or anything of that nature. He saw the bundle of armor, of course, but that doesn't mean he noticed the ring or that the gauntlets were different. Once the armor was tied up, whether the gauntlets were visible or not is unknown. The blacksmith, on the other hand, did examine it. I can think of all sorts of possibilities for him to prevent the ranger from seeing it at that point.
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)
you'd be surprised at how many things in plain sight that we don't see.
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.