Except that isn't what happened. The DM didn't pull a gotcha. Rather, he just roleplayed out the consequences of forgetfulness. If you don't want that to happen to you, don't forget things. It's not the DM's job to be your memory.
We've been over this, particularly since more information was added, such as the ranger actually showing sufficient interest in what was being sold as to make an Int roll to appraise it. I've also described how the shopkeeper's language was not the 'tip off' it was originally thought to be, as suits of armor are, in fact, sets of discrete items.
As you concluded last time, you felt, for some reason, that it was the DM's option to whether the player gets a perception roll to see the incongruence between what he thinks he is offering and what he is really offering for sale. Despite the fact he is almost assuredly handling the items in question. Despite the fact he is making an Int roll to appraise them.
Why you feel this way is still not clear to me. This isn't an issue of whether it is the DM's job to 'be your memory'. It's an issue of whether it is the DM's job to be your eyes into the world. (And I strongly feel that it is the DM's job to be your eyes into the world.)
I have maintained the idea that this seemed to be a DM error, and that's all fine and well. But, let's think about what you are offering for a moment, though. You're proposing it is acceptable to purposely:
(1) Focus on a mistake in a previous scene
(2) Purposely deny the player sensory information in new scene as he handles items
(3) Totally ignore a new action by the player which absolutely requires sensory information
(4) Impose a 'consequence' on the players based on above
Done purposely, I can't see how this isn't a 'gotcha'. Can you clarify this? Do you not feel the DM's job involves being the player's eyes into the world? What's the difference here between us?