Henry Wiencek said: I'm interested in tracking the independent, money-making operations of slaves at Monticello.  
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"...Among his (Jefferson's) slaves were artisans of all kinds.  The bricks for Monticello were made on the estate.  The nails for the mansion were wrought by slaves; indeed the sale of wrought nails was one of the sources of income for the estate."  

From:  American Statesmen: An Interpretation of Our History and Heritage
By Edward Howard Griggs, Published 1970 Ayer Publishing

But with regard to 'independent money-making operations' I suspect these will be difficult to find.  (Hopefully, Dr. Kukla's research will be fruitful.)  However, in the few cases where a slave's artisan operation was even allowed at other than the slave's 'home' estate, the income was collected by his 'owner' with sometimes a token amount for the slave to make a minor purchase (from my admittedly limited studies.)  Wood barrell-making for shipment of tobacco was likely the most common craft at these many plantations where it was extremely difficult for a field-hand to develop 'any' artisan skill.  I recall from a visit to Monticello several years ago there being several rooms on the lower level and exterior areas (furnace for example) where various crafts for maintaining the estate's operation were conducted.  I don't recall any 'independent' money-making operations being obvious or even mentioned, everything was for the estate.  It seems there would have been very little stop-by 'retail' traffic; any significant income must have come from 'bulk shipments' by the estate, such as wrought nails, I should think.  Might Jefferson's 'purchase' of the soap simply been a gesture, or for a specialty soap having a particularly likable scent, I wonder.  As you say, the estate had a large soap-making operation.  And how were slaves to be paid since there was no 'money' until several years after the RW, except for rather rare pieces of Spanish coins, and barter/account-keeping was the means for most local commerce, such as Martha's bacon for poultry.  Even labor by white and freedmen was commonly bartered in pre-RW times, at least in frontier areas of which I am most familiar.  

Btw, Benjamin Franklin was son of a candle-soapmaker in Boston, the trade in which he was almost 'saddled' after leaving formal school at age ten to apprentice with his father, before moving on to greater accomplishments.  What if.... 

Neil McDonald