My own eyes, and I don't know why that's even a question.
That's irrelephant.
I don't understand what point you are trying to make here. By the way, I did not read 'irrelephant' as 'irrelevant' - my immediate reaction was 'what does irrelephant mean?' and I actually went and googled the thing to see if it was some word I didn't know. Apparently there is some slang or meme thing I don't get about something being unrelated to elephants, but again, I don't know how this relates.
Anyway, I am going to lay out my position as clearly as I can in the hopes of having some understanding.
Basically, the rules seem quite clear to me - since you can't gain another copy of the last card (be it Sir Martin, or the last card in any 'normal' pile, or whatever), you can't be 'about to gain' that non-existent other copy, which means you can't trader it into a silver. Seems very straightforward - you can either get one copy of the bought card or (assuming there are enough silvers left) one silver, not two of either, and not one of each, which is what you'd be able to do in the case of multiple cards being left.
Nothing you can say here is going to change that. You can change the rules of course - you are in a unique position to do so - but if we're going to play with the rules as printed, then what matters is what the rules as printed say (which is what I was equating with 'my eyes'), and not what you are saying here ('which is what I was equating with 'you'). Of course, we can always play with some other rule set, as people do all the time - not forced to throne room things, for instance, or with point counters or notes or simulators or what have you. But as you are so fond of saying, these are variants, if that should ever matter, which hey it's just a card game so it doesn't really.