In my opinion, already everything is very cheap - the game is very easy, It does not matter if you lose a few silvers for add the item (the tax is already paid when collecting the package from the post office, the tax after adding item in auction is for the possibility of popularizing your good)
If you want make any "tax-changes"... I would be more interested to make refresh with the value of items so that the tax is adequate - in some cases the differences are very high.(Chees for about 50-100g? Elixirs with prices several times higher? Skulls with 20s price?) There are many examples of wrong prices, which caused the players to lose few silvers, or even golds
I just wanna be famous...
...but be careful what you wish for