i think something like the Runescape grand exchange would be good, where people can offer cards at prices within a certain range. (i think its plus or minus 5% of the market price, could be different for elements).
selling at a low price means it'll be more likely to sell quickly, high means you get more profit if somebody cant buy it low and goes high. a bunch of transactions at a low price means that the card's market price goes down, one at a high price means it goes up. the thing is there'd have to be very strict limits to how high or low certain cards could go.
for example:
no more than 150 or less than 10 for an unupped non-rare, no less than 2 or more than 15 for an unupped pillar, no less than 100 or more than 1000 for an unupped rare, plus 1500 for the ranges of upped cards. then cards would be valued on how often they were used, not values given by Zanz.