The problem with your idea is that if your group gathers 10 mil and offers that for 100 TBT, and John McHacker is offering 20 mil for those same 100 TBT, people will buy from John McHacker unless they just don't see his shop. Also, while having several people gathering bells makes it easier for everyone involved, splitting the TBT payout means each person doesn't get much for his / her efforts. The problem with any form of self-regulation is that those who choose not to self-regulate can very easily take advantage of those who do. The only way the price of TBT will ever fall is if mods intervene. Aside from price fixing, I cannot think of anything off the top of my head which would solve the problem.
People are greedy so they will take the best deal. I mean, I would. There's nothing we can really do at this point.