An interesting point I've never been able to find figures on. If you have any, I'd be really interested in seeing them.
I'm thinking they aren't the sort of figures for which people would want records kept. This is the sort of thing one does in the dark so as to avoid the moral opprobrium directed at himself by his neighbors and fellow abolitionists.
Some of it could possibly be reconstructed from existing records, but I think a pretty good statistical reconstruction could simply be had by assuming most previously held slaves were sold in the South.