I don't really like the idea of having the entire deck open. The first person to get Black Market out would be able to buy the best card that nobody else could get (Mountebank, Ambassador, what have you).
One idea might be to allow the Black Market stall to "stay open." After someone buys his card, the remaining cards stay out. The next person to buy from the Black Market can choose one of the remainders or pitch them and draw 3 new cards. This, unfortunately, would actually make it unfavorable for the first person to get there. Perhaps that can be offset by starting the game with two Black Market cards flipped over. That really is more of a Black Market variant and not a Black Market deck.
One possibility could be to have the Black Market deck contain all of the trashing cards. Make people work at trimming their decks down. It'd be interesting to see if that pays off or if someone does better focusing on the main board rather than a delayed trashing.
You could have cards that cost $6+. Then, it truly would feel like a Black Market. The card actually would be less useful early in the game unless you think you can whip out $6 in turn 3 or 4. Peddler, however, couldn't be part of it, unless you made a house rule that the Peddler's cost is reduced during the Action Phase (or the Black Market's faux Buy phase).
By contrast, you could put all the $2s and $3 if you want to mimic the shifty street vendors who sell things out of the backs of vans. That could be kind of fun, as you would buy a cheap card with Black Market and then get your regular purchase as well. If you want to really be perverse, have all cards on the board cost at least $4 (except Black Market). Then, you can only get the smaller stuff if you have Black Market. Though, if you have some tasty Wharves or Laboratories out there, you might just skip Black Market altogether.