Little late on the bandwagon here, but I'll have to agree with having Yujo defend the bijuu until his account is officially sold. If he is under the impression that his account is going to be sold before a formal challenge can be initiated and completed, then I believe his current Kage should be responsible for taking on the burden of the bijuu.
This means that if Yujo decides to continuously ignore Luka's challenges, then Luka can fairly challenge Tsuyo for it. But from the looks of it, it seems that Yujo will be accepting the challenge after all~
Edit:
But yeah, just to keep this conversation related to the actual topic at hand, can we make a "rule" against openly selling the Bijuu/Items for money? I know that there isn't any failsafe way to prevent this (and if someone really wants the cash they can find a sneaky way of making deals), but can we at least make people less obvious about it?