Prosperity and S/D are linked.
S/D is the amount of goods that can be sold in your fief.
Prosperity is the amount of S/D you gain PER DAY.
If, S/D Goes BELOW Prosperity, then price defaults towards average(which is 15).
Thus if you wanted to be SUPER picky whore about it, you should NEVER sell below Prosperity because it takes 1 day for price to Reach what it was.(of course you can then SELL at that price and repeat the process, but doing it without going below gives you just that little bit of EMERGENCY that you can sell in a pinch.)