Potentially 12VP for 4 cost is a little high... What's the max Fairgrounds can reach without Black Market? (11 Kingdom piles, 10 differently named knights, 5 prizes, 5 different named ruins, 3 shelters, madman, mercenary, spoils, 7 standard supply piles, platinum, colony, potion...
)... That's 46 different named cards in a game without black market, giving fairgrounds a max of 18vp at 6 cost (the average max is probably closer to 6 or 7vp)
Having this card available will either cause very few provinces to be bought, or cause provinces to be bought in defense. Playtesting will tell which is more likely... (Or let the puzzle forum have their way with it...)