It started off as a $20 card, before dropping to about $5
I personally really like Thoughtseize because it always feels great to be in control and trade your one-mana spell against some big threat of your counterpart
Almost instantly, all the sets products sold out everywhere, which is practically unheard of for MTG
Curry favor by rushing down the combo or control player, then use the threat of burn spells to encourage other players to do your bidding
So he gets a fair amount of protection from that
Looking back now, 5 mana is an awful lot to ask of an enchantment that doesnt do anything when it enters the battlefield, but if allowed to sit on the battlefield after a board wipe or a couple turns of play from an Aristocrats strategy (decks that derive value from sacrificing creatures), Black Market can quickly generate game-winning amounts of black mana