I've done that too! It's a great way to simplify the "haggling"/"selling" process into something easy for players to grasp and quick to resolve. It's quick because you either manage to find a specific buyer and get the good price, or you don't and get the market price -- there's no temptation to keep making Diplomacy checks in an attempt to eke out that extra 5% profit.
Actually, why dispense with the Diplomacy? Maybe they can convince him to pay a higher price, but it might require them to use other skills or actions to convince him. What I'm talking about is a skill challenge, where the reward would be a minor parcel as far as you are concerned, but it would be presented to the players as an increase in the buying price. It could be in the form of helping him out with some other tasks, or maybe a skill challenge that isn't directly involving him but has indirect results which might have an impact on the price.
What do you think?