"Market" might suggests off-class Asset shenanigans and you start planning team gymnastics in order to use this card, which is fun. But it is also simple to enjoy this card without extensive strategy meeting : Just check out your friend's deck if they contains Enemy Phase / Mythos Phase reactive Fast Event cards or not then you can decide on your own to purchase this card. (Effect lasts until your next Investigation Phase.)
It basically widens the card's coverage to the max. Sometimes that card can only be used on self, then Black Market turns it to anyone. Sometimes that card says you can use it for friends at the same location, Black Market makes it even more flexible who would pay the card's cost. Also they became open information, not often you get to say WE ALL have Lucky! / Ward of Protection in Investigation Phase.
5 cards is quite a wide search in a thin deck game like this (5/25 in the first round, but after some time had passed you have better odds.) Check that player's discard pile if what you are aiming for are gone or not before going for it and hope they are not on their hand. Some examples from the Revised Core Set :
- Lucky! : Ultimate team-wide "insurance" for Mythos Phase, since if players prior tests and pass normally this card would stay for the next player that draws encounter. It is even more flexible than Lucky! (3) that requires same location.
- Dodge : Already can be used to a friend at the same location, now it works remotely as well. You can also use it right now in Investigation Phase to bypass AoO to do things, or try a risky hit against Retaliate. Some encounter cards order enemy to attack an unfortunate investigator (like checking Prey, or lowest-something, that is not the player) now anyone can stylishly dodge that.
- "I've had worse…" (4) : Valid complaints about this card that rarely something higher than 2 lands on the player to justify Lv. 4 version. Now that it covers everyone's Mythos Phase draw, much higher chance to nullify nasty fail-by on someone's weak stats and get 3+ resources. (Or to deal with enemy attacking/Retaliating 2+2.)
- Ward of Protection (0) : Upgrade to almost-Ward of Protection (2), now it does not matter who got Ancient Evils. (But that player must pay small price of 1 resource + 1 horror.) It is even better than the real Lv.2 version if you want to cancel Revelation on Peril card!
PS: "One at a time, reveal cards from the top of any investigator deck(s)" : You can even stop revealing when you found your desired target and switch to someone else's deck for the remaining cards. The more colorful your team composition is, the more choices you can cook up the market to perfection.