Ixon does let other people from the guild use items. *shrugs* Not the ones that other people have donated to the museum, as that is not his place, but the ones he has bought himself - yes. He used to let other people have them but after a series of people a) leaving the game without giving them back, b) accidentally destroying them or c) pretending to lose them and keeping them stashed in their bank for months on end, he made the decision to keep it guild only. The nature of Twenty Two is such that we are close enough to be able to share rares without fear of permanent loss of an item.
As for the mercenary swords and their 75 plat value, these are not unique items. That quest has been run at least twice, and probably will be run again. Hence the lower (yet still high, for what they are) value on these weapons.
Yes, the museum started because Ix loves to collect. No, of course I'm not saying that it is 100% altruistic - very few actions actually are. He takes enormous pleasure in owning or care-taking unusual or rare items, but also equal pleasure in safe-guarding the history attached to them. Yes, he has been smitten before for the actions you described, but that was before he married me and now he wouldn't dare do such a thing. The wrath of gods and fear of the wrath of his wife have taught him a valuable lesson.
As Danica said, the value of an item is not determined by its material or its purchase cost. It is determined by its place in history. An almost worthless scroll to us is -was - worth the equivalent of 2500 plat. If it is greedy to wish to possess that in order to mark it with a place and description in this museum, then I too am guilty of that particular sin.
Of course we love collecting (I include myself in that, and feel qualified to write this because many months with Ix have taught me much about both the rares business and the history of Trinald) and enjoy possessing items that we view as special for whatever reason. I make no apology for it. But the reason we do view these items as so special, and the reason Ix devotes time, plat and energy in purchasing for and referencing the museum, is because we care deeply about Trinald, its past, and preserving that for the future.