One more thing: following my purchase of "goods" the fief did not get the price I paid. It may go to the fief owner, which is obviously "0" atm, so it's hard to say. I am not sure about tax, as it was only 0.15, and the cash in the fief raised by 1.
EDIT:
Once you purchase some goods, you cannot buy some more in the same location, even if you leave and enter again, only SELL option is active. I guess it works if you move to other location as well (to be verified). So forget about traveling around buying and selling some stuff in various places; you need to buy in one place and sell in others, and you are unable to buy more unless you sell everything you have.