A better strategy would be to estimate card value based on your starting hand (i.e., probability that each suit is goal suit). It requires some non-trivial combinatorics but I guess if you take the game seriously you just memorize those once. Then you buy cards if the price are lower than your estimate and sell if it is higher. You also should pay a somewhat more for the card that gives you majority (4th/5th).
The updates are in this case, at least assuming others are doing something similar, is to increase expected price if you see sell for higher price and reduce if you see the sell for lower price. To figure out how much to update would be the hard part.
The updates are in this case, at least assuming others are doing something similar, is to increase expected price if you see sell for higher price and reduce if you see the sell for lower price. To figure out how much to update would be the hard part.