I always play with the entirety of the remaining cards. One thing I like this about this method is that everything is fleeting in the Black Market. With over 100 cards available in Black Market, each option you receive will likely not be seen again.
Although, I'm intrigued by the idea of making a 15-card or 25-card deck. As the deck cycles through, you can plan for future cards. That actually adds a new level of strategy.
I'm leaning more toward a smaller Black Market deck now. Another good reason is that setup is easier. Right now, you'd have to put a token on all victory cards just in case someone buys a Trade Route. You'd also have to get the Prizes out in case someone buys a Tournament. And I can't even remember what the rules are for Young Witch, but I'm guessing you'd have to choose a Bane card. At least if you draw the Black Market deck, you can perform those setup steps as they are needed.
How would I choose a Black Market deck? I'd randomly draw the 15 (or 25 or whatever) cards. I'd cast out cards with a Potion cost unless there was already a Kingdom card with a Potion cost; I believe this was listed in the Alchemy rules, but it's been a while and I can't verify that at the moment. But, I doubt anyone is interested in buying a Potion in the hopes of getting a card or two from the Black Market deck. If that's the case, then Potion cards are just dead space in the Black Market deck.
Come to think of it, I've vetoed the use of Black Market more often than not, exactly because I wasn't in the mood to deal with it at the time. Perhaps if I restricted the Black Market deck to a smaller subset, I would not be so eager to choose the path of least resistance. I'll consider that when I play tonight.
Kevin