Sounds like the mercantile skill on Morrowind 
Indeed, and NPC's are the kind of people I want to trade with last.
I was thinking, maybe it would be an idea to link the quantity of talking to the training of charisma. Trades happen to talk a lot, that should work directly at their charisma. Just like running should be linked directly to stamina, and carrying stuff around linked to strength. That way you cannot train your strength in two minutes from 50 to, say, 100.
I don't know if it's doable, but it's an idea.