This comic was a late thought I had, and I think it's an interesting question, so now this comic exists.
By the logic of how trades are supposed to work in the comic's canon, I think the answer would be "Age" (Nanase gets younger) were it possible. I ALSO think, however, that it wouldn't be a valid trade in the first place.
Of course, this is just me saying this. Commentary Dan. Anything I say is up in the air until confirmed or disproven in the comic itself.
Me saying it's not canon until in the comic isn't a philosophical thing. It's just practical. It's 100% possible I'll say something in a commentary that's "new information" only for a story idea to contradict it later. It makes sense to look at is as "maybe-canon."
Tangential rant involving ice cubes
EGS isn't opposed to the idea of things having some sort of magical value that can be traded even if it doesn't really make physical sense. That's basically what the trading system is in a nutshell.
"Youth" seems to be a transferable thing in a lot of fiction, with machines and magic that can steal it.
From a story perspective, that makes sense. Increases to longevity and feeling young are things people want. Give them a means to take it from someone else, and you have wretched villainy with an understandable motive.
The thing is, however, I think of youth as being a bit like cold in that cold isn't a thing. Cold is the absence of a heat. I think absorbing someone's youth makes about as much sense as "absorbing cold". Ice cubes do not work that way.
But, again, magic can have its own logic, and it probably SHOULD have its own logic, because magic should be weird and reality defying. "How the heck even, that does not match up what we know from apples falling on people's heads" should be a thing with magic.