In the 3 Monster Maker guide that I'm writing I've find that it is better to let the inventor go to the store to sell his own stuff after he is registered as an inventor.
His companion (sort of house cleaner slave) was also registered as an inventor after learning one skill of it. Than I did some test in letting him sell the stuff my 'real' and higher skilled inventor makes. It remains the same that the companion remains to sell $30,- of inventions, even though he sells it for more than $100,-. When my inventor sells his own stuff, he was able to sell it for a higher price, even though his companion is a born salesman, and the inventor is not.
So unless I have forgotten to investigate something, it turns out for me that it is better for a sim to sell his own stuff. Especially when he is more skilled in the particular skill.