im not sure if this was suggested before here but i read it somewhere from someone i dont remember so sorry for not being able to give credit
Zeni tax should increase in my opinion too so it will keep people from selling for insane amount of zeni and if they do they will be risking 10% of their zeni
And private shop zeni cap should be decreased in order for this to work to 1kk-5kk something along the line.