In fact, I am fairly confident that in the rare situation where you have so may cards in the black market, even if not all the cards you own, that there aren't any 2/3s left, or the more common situation where people aren't playing with that many different sets - say I'm playing with just base and cornucopia or something - that by the rules of the game, there's no other interpretation than to have no bane. It's very similar to the 'well, if curses are always available, there must be something to do with them' kind of mindset, where you get people to just come up with weird house rules because that's all that seems to make sense to them, when really that's not what the game does.
Again, I don't really expect this to come up very often. I just don't see how you can possibly materialize your other solutions from the rule book, given that they're just not there.
I actually don't see how you're reaching this conclusion. Let's follow the rules and see what we get:
1. Black Market's in the kingdom! What do we do?
—Make a Black Market deck out of one copy of each Kingdom card not in the Supply. Okay, in they all go.
2. Hey, that means Young Witch is in the Black Market deck! What do we do?
—Add an extra Kingdom card pile costing $2 or $3 to the supply.
3. Okay, where do we find this Kingdom card pile?
—Well, some $2 and $3 cards are in the kingdom already; obviously can't use those, since they wouldn't be "extra".
—Looks like we'll have to choose a $2 or $3 card that's not in the kingdom already.
4. All right, I found one! Wait a minute, there's only nine cards here.
—Oh, right, the tenth one is already in the Black Market deck.
5. Well, Young Witch says we have to add this Kingdom pile to the supply. Kingdom piles have ten cards in them (or eight or twelve if they're Victory cards). So go get that tenth card out of the Black Market deck and put it back in the Kingdom.
Note this argument has interesting consequences in a two-player game if Tunnel or Great Hall is the bane.