Wowhead News has more information on MoP's Black Market Auction House. Check out their post for all the details on where it's located, the costs, etc.
What I'm more interested in are the possible items sold on the BMAH. I spy with my little eye some rare pets!
Remember that this system may still be incomplete and might not be set in stone yet. What's previewed on the beta might not be the final version. And yes, I feel this warning is warranted since some might get a bit miffed about a few of the items being sold on the BMAH.
General Pets For Sale
So far Breanni looks like she's found a supplier for most of the Argent Tournament pets. She's also selling some (might not be all) of the pets that can also be found from the random dungeon Call to Arms reward bag, Satchel of Exotic Mysteries.
I'm not entirely sure collectors are going to be all that willing to drop 10,000 gold on either grouping of pets, though.
The way I see it is that mainly seasoned players will have an excessive amount of gold to spend, but these older collectors will either already have these pets or are more savvy and can manage to acquire the Argent Tournie pets and miscellaneous companions through other (cheaper) means.
Newer collectors will probably find the BMAH prices too high and stick with the Neutral AH or even the basic AH for these two categories of pets.
Some collectors will take the traditional route and farm for these pets on their own.
This is all just speculation, though, and I'm sure some will be quite thankful that they won't have to travel all the way to Icecrown every day to complete the tournament dailies. Server prices vary as well, so who knows? Maybe the BMAH will be cheaper than the Neutral or regular AH on some servers.
Still, 10,000 gold for an Obsidian Hatchling? Breanni really overpriced that one. :P I don't know how much effort it takes for some players to get to Dalaran these days, but it can't be 10k gold's worth!
I've gone over the basic list of possible pets found on the BMAH, so let's discuss what might be the more controversial list: TCG pets.
At the moment there are only three TCG pets that might be sold on the BMAH, but they're especially rare. THESE companions would be worth the 10-20k gold (starting price), for sure.
On the other hand, many collectors who wouldn't normally have a chance at the rarer TCG pets finally have a shot at acquiring them.
Now it's just a matter of how fast you can reach the gold cap, as there is no buyout option or bidding cap (that I'm aware of). This might mean the winner will ultimately be who can bid the max amount of gold the quickest.
Also remember that the pets for sale on the BMAH will be on a possibly random rotation, so it could take months before a TCG pet pops up. It might not even be the one you're looking to buy. Collectors will be playing with the odds here, and there's no guarantee they will win.
It's the same with many auctions in real life, however there's a higher chance for a definite win when dealing with sales on Ebay or other websites. At a higher cost, of course.
I suppose it's the best compromise if you think about it that way. You can either take your chances in-game with the RNG of a specific TCG pet showing up for sale and attempt to be the fastest to bid PLUS be the highest bidder, OR you can spend real life money for a card on Ebay but feel slightly more confident that you'll win it.
In the end, I don't think the BMAH will make or break pet collecting. It will make a few pets slightly more accessible, but it's in no way a guaranteed or efficient source for companions. Collectors will still be trucking along in the same way that we've been collecting for years and years - hunting, grinding, and waiting. Oh, and can't forget capturing come MoP! :)