A solid in-fiction explanation of what this mechanic represents would go a fair way towards selling the concept. If the XP is being bartered between two players, what are their characters talking about? What would it look like?
"I spent three days beneath the Monolith, hiding from the cannibals that killed my friends. Three days of fear and loss. They can be yours, for the right price. I know I don't want them anymore..."
Maybe a Weird move that allows the transfer? If it carries some risk of the MC making a hard move, the players might think twice before engaging in casual use of the mechanic.
Also, if it's established that the PCs can trade this sort of thing like a commodity, what about NPCs? If they can as well, is there a concern that a relatively rich PC can start to buy XP by bartering their oddments in the marketplace? Fiction-side, the NPCs are out there learning, surviving, getting by. Rules-side, NPCs don't accumulate XP. You might need guidelines for how many "loose" XP any given NPC might have lying around.
I think it's a neat idea. It would really affect the fiction of the game, I'd imagine.