S0AD
If a marketplace tax was that high people would just sell in exchange.
Ahem, they already do.
The marketplace is only used by venders who want top dollar when selling.
... and by those who use the marketplace to show off or hold an expensive item.
(Albeit that situation could be avoided by adding a percentage fee for listing an item)
(A 1% listing fee, for example, would cost 9.9T to those who put an item up for 999T)
However a listing fee would be just as much as a deterrent as higher or new taxes.