I'm not sure if this is a bug or not, but it's not what I expected:
Playing in a game with Capitalism, and having bought it, I then had multiple Black Markets that I waited to play in the buy phase. I played one, and then a second as a treasure before selecting my purchase for the first BM. When I played the second, another BM selection of three cards pops up. I then played a third BM, and the same things happens. This is actually very cool, because I got to see all the BM cards for all the BM plays before choosing any to buy.
So far this seems reasonable. But then after I played my last BM (I think I had 5), I then played my Fortune, thinking, great, I can double my money before making my BM purchases. But when I played it, all the BM cards went away, as if I had selected not to buy anything from any of them. I was still in my buy phase, with my money doubled, but I couldn't make the BM purchases at that point.