I do not see the problem with mass listing a bunch of items for a small amount of gold. Going back a step, how would a player post an item on the auction house if they don't have gold to begin with? Players who usually put items that are low in gold price are players who are poor in gold and want to get some quick gold, or the item itself does not cost that much. For example, if an item costs 1 gold on the auction house, but due to the tax, it is forced to be placed on the auction house for 6 gold? Would anyone pay that extra 5 gold to buy that item, when the item costs 1 gold?
This tax system is implemented to assure that people only sell valuable stuff like Ranks, keys, etc. on the auction house, rather than cheap and small stuff like kits, axes, etc.
By adding this it is made sure that small things are sold for ingame money and so new players can buy stuff too just by grinding. Last season, when there was no tax, the auction house was full of stuff listed for gold, no matter how cheap the thing was and so there was no purpose of the ingame currency besides buying spawners. No Tax feature was making the gold less valuable day by day. Even new players got gold for selling stuff worth small. Your point that who would pay the extra 5 gold on an item worth 6 gold is correct but once again, this system is implemented for valuable stuff worth 100+ gold to be sold on the auction house and so no one will be able to sell something that small and so they should use the ingame currency to sell it.
In my opinion this tax feature is a good thing, while changes can be made, I do not think it should be removed.