they should just add taxes for buying AND selling
everytime you change a price for an item your retainer sells, you need to pay like 0,5-1% of listed price
on the long run, some items would not be so expansive cause the seller would ned to pay more taxes, and many items will not drop so fast in price cause there will be less undercutting in general
and does fu**ing undercutting and crafting bots need to pay huge amounts of gil to list there items first every 30-60min
but i like this blind market idea too
the Steam market also works that way