Hmm, I'm gonna throw my idea out there.
I think everything is fine the way it is, EXCEPT for the rate of exchange with the ACNL bells. There should just be a set amount you're allowed to trade, lets say 1 mil ACNL = 100 TBT. It wouldn't take anymore work for the mods than to put a sticky that says don't sell for more than that. I think they wouldn't even have to enforce it really, because the community would self-monitor it, and it would just discourage people from continuously raising the price to crazy heights.
Why do you think people can keep raising the ACNLB -> TBTB rate?