I'm staying on a group of islands off the Georgia coast that claim the last shipment of smuggled slaves arrived in 1865. It has piqued my interest about slave smuggling. Who was involved? How profitable was the trade? What happened to those who were caught?
You could look into the De Wolfe family, of Rhode Island. Slavers, congressmen, financiers of the Episcopal church in RI.
The New York Times just came out with an article on Rhode Island's role in the illegal slave trade:
There is a great documentary from a few years back, which won awards and I'm pretty sure was played on PBS, called Traces of the Trade, which also follows the De Wolfe family. My favorite part is when they get a map of the family's holdings in Cuba post-Civil War and figure out that they still owned slaves, but in Cuba instead of in the US:
http://www.tracesofthetrade.org/
An amateur historian named Cynthia Mestad Johnson wrote James Dewolf and the Rhode Island Slave Trade based on primary sources in the Rhode Island archives. That book is on Amazon.
I'm no specialist in the illegal slave trade myself so I can't give you more than that. Sorry. I just know about these guys because they had links to Cuba, which is my field of study.
Edit: Damn, I read your principle question but not so much your follow up questions. Here goes:
Who was involved?
Directly, major figures in much of the US, including the North. The De Wolfe family was one of the single most important families of its state and were able to do so with impunity. I'm sure their case isn't the only one.
How profitable was the trade?
Profitable enough to make the De Wolfe's one of the richest families of their time. The wealth also found its way into the local economy where the slavers lived. The slavers would buy irons from local blacksmiths, rope from rope makers, food stores from local farmers, etc., and thus these would indirectly partake in the immense gains that slavers would make.
Eric Williams spoke of a similar phenomenon occurring in English cities, like Liverpool, in his book Capitalism and Slavery, but with the difference that it was at least legal when English slavers were doing it in the 17th and 18th centuries.
What happened to those who were caught?
On this point I am totally ignorant. Perhaps a specialist in the American slave trade could answer this better.