Yup, you read it right, a "black market" for mixed breed rescue pups.
This must seem INSANE to the many good people 
on this forum who are saddened because they know healthy, young adoptable dogs being euthanized in their local shelter---- but according to a newspaper article, the demand for pups is so high in New England that not only do rescues and shelters transport pups, private individuals are bringing puppies here in large numbers.
I don't know if this trend will continue---there are an increasing number of dogs with behavior problems winding up in the news or in a local shelter because of groups/individuals that do a poor job of screening the dogs/ adoptive families/ and following up.
I am going to link to a couple of articles, but I don't know how long the links will be viable so I'll snip a few bits:
Rescue dogs from South (not this one) become local 'horror stories'http://www.seacoastonline.com/apps/pbcs.dll/article?AID=/20080529/NEWS/805290424
Here's one: More than 50 percent of all the dogs at the NHSPCA shelter in Stratham come from shelters in the South and the West.
Here's another: One pet transport company, PETS in Cookeville, Tenn., makes runs from the South and Southwest just about every week. It is used by Jody Merrill of Portsmouth, who runs Loyal Heart Puppy and German Shepherd Rescue and regularly brings dogs to New England homes.
Merrill said in each PETS run, 50 dogs come to its drop-off in Windham, N.H. "Say they come 42 times a year. You do the math. And that's just one transport system."
And, said Merrill, because there is a dearth of puppies in the Northeast due to the high spay/neuter rates, she estimated more than 90 percent of all puppies in local homes weren't born here.
ONE transport brings 2,100 dogs a year to Windham NH? That is mind boggling when you consider the size of the towns in that area----the HUMAN population of Windham is about 12,000 and the largest town in the county is about 36,000.
NINETY PERCENT of local puppies were born outside New England???
Animal officers: Rescued dogs from South proving dangerous Potential owners advised to do research http://www.seacoastonline.com/apps/pbcs.dll/article?AID=/20080528/NEWS/805280371
YORK, Maine — A rottweiler attack of a puppy on Friday is symptomatic of a little-known but potentially serious problem: rescue dogs with behavioral issues living with owners who are overwhelmed by the necessary care.
York animal control officer Larry McAfee said the three or four cases this past winter in which he had to invoke the state's dangerous animal law all involved rescue dogs.
These dogs have often been through trauma of some kind and are frequently placed online for adoption, only to find a home with what one animal control officer called "Animal Planet people" who simply don't know what they're in for.
No, they're not knocking Animal Planet, this is what they mean:
"They're what I call 'Animal Planet' people. They love animals. They're concerned when they see these dogs on a show," he said. "But then, miraculously, 15 minutes later, the dog is catching Frisbees in the back yard. What they don't see is the two years of hard work and training it took to get the dog to that place."
BTW The article started with a rottie but it wasn't written to highlight certain breeds, here is the other example:
A month ago, an adult Labrador retriever from Kittery Point was loose on Long Sands Beach when it attacked five Yorkshire terriers and their owner, McAfee said, although no one was seriously injured. The Lab's owner told McAfee that she was the dog's fifth owner, and it had originally been rescued from the southern United States
FIVE owners??? 
While I'm glad that adoptable dogs are being saved, I'm also concerned about the potential implications if this trend becomes permanent.
Saving a puppy sounds great----but what does this mean for local adult dogs that need homes?
What will happen if there is a dramatic increase in the number of young adult dogs (adopted from out-of-state) dumped at shelters? Will New England shelters find themselves having to think about euthanizing for space like the southern shelters? Are we just shuffling a problem from one part of the country to another?