Hazama and Warren both make valid points, and since this is based on personal opinions i'll go ahead and give my own. Keeping it strictly professional, a bijuu is indeed a possession to be passed around, or hoarded; a rental if you will. If Bocc wishes to sell his, it is just another way to lay claim to the beast for a person who may not have wished to fight bocc. However, I don't believe that in this process he should be able to deny challenges for it either, until it's bought. I don't know if he is truly quitting considering he said in the mission hq that he's done with serious RP which implies that he'll go into Raifudo lurk mode. As of my Nibi extraction, i'm not brushed up with the rules of bijuu, which I should change in the near future, but I must say that there isn't any rule against sale of the bijuu to my knowledge.
Now personally, it should be a rule against it. Buying a bijuu simply isn't...traditional, for lack of a better term. Bijuu are trophies in a sense that should be earned, and placing monetary value on them cheapens that, also insulting anyone who's gone through the crucible of having to fight for one, (which on average a bijuu battle lasts for say a month+). With some fights, they are grueling experiences, and for some they are decent challenges. Having to fight Raku/Bocc, it was a long fight, but it was good in general so I can't personally speak on behalf of those who went through the awful but the point is still there. This should be addressed promptly.
If one has to choose who it goes to, then that inheritor gets the challenge queue, and all, though it's also unfair for a challenger to have to wait 2 more weeks while getting played in a game of monkey in the middle.