I was playing Malcom and he had a want to make a sale to a particular sim. So I had him make a Basic Sales pitch. Later the sim bought the object, and Malcom even rung them up, but didn't get the want fulfilled. I don't know if that's a glitch or something I just didn't do right. I'll have to look in the Prima Guide to see how to fulfill that want. I'll keep an eye out on his wants too.
As far as I have noticed, the Sell something want on a specific sim, means you have to sales ->one of the options, and fill their buy bar, so they boy the item as a result of your sales pitch. Just doing a basic sell, and then wandering off, without the bar filling, wont fill the want.