So, if I've read this section of Black Markets correctly, the more people involved in a black market, the harder it is to find out about it? Huh?
I assume there's a good balance reason for making discovering a tiny hideout where a mere thirty people trade illegal goods is only a DC 12 to track down, and a massive underworld economy where literally thousands of people (most of whom can't realistically have rolled the DC to find the market they're participating in) require a DC 30 gather info to find, but it makes no sense at all.
The table would make a great deal more sense if the population column were inverted - if you just need to move some stolen goods, you can easily find someone who will buy it from you (thus thousands of members and a low DC), but they can't deal with high-end stuff (thus the low purchase limit); if you need to move the fantasy equivalent of a WMD or priceless piece of art that's instantly recognizable, there's only a few people who can move that sort of thing, and they're not easy to track down (low population and a high DC), and they're able to deal with even the most expensive of items (high purchase limit).