As example invest more of the Cards in booster. Why giving them a card to sell instead of a booster? Like u are getting Frost-Mage but yours is already full. Would u realy be happy about that? Or you get a Thunderwagon(R) but don't touch fire, I definitly would prefer a Booster because of Gambling or sell booster. Also that would make it easier for price Pool like 1-3 getting special prices, 4-10 nearly same amount of boosters and maybe something special and 11-20 the same+all the others the same like a price that everyone gets that take part.
With unsold cards u can make Give Aways duri