About the fee, imho, a better solution could be applying it only on listings that are updated more than x times a day (where "x" could be equal to 1).

If I list a card for the first time in a day, I shouldn't be charged with a fee.
Same if I update its price the day after.

On the other hand, if I want to update its price or re-list it multiple times per day, a fee could apply.

It would also be ok to have a free list/update per card every 2 days (similarly to the minimum rental time of cards).