I thought that you could pay off debt in B1 Black Market, but you can't play any other treasures afterwards?
(other than in non-standard ways like with the Storyteller that you bought in that Black Market and that gets played with Innovation)
In other words, the transition from B1 to B2 is not something that needs to happen on an empty stack like moving from A to B. But once you buy something / repay debt, you can't play Treasures / use Coffers anymore.
I think I've ruled that way, but now that is seeming overly confusing and fixable.
First you can play Treasures and convert Coffers, then you can buy cards and pay Debt.
Playing Treasures can make all sorts of things happen. Buying cards can make all sorts of things happen. Those things aren't what the buy phase is about though; the buy phase is about "you can play Treasures and then buy a card," and then +Buys let you buy more cards, and expansions add Coffers and Debt and locate them in there.
So, you can't decide "now it's time for the buying cards portion" in the middle of resolving e.g. Capitalism'd-Storyteller, in order to pay Debt then.