One tweak: I think there should be a min sell-tax. Why? Sellers of single sub 10-20g items don't pay any taxes due to rounding. (I presume you essentially sell for free at up to 10 or 20g?) I just don't 'get' the people who waste time and retainer slots on selling 1g items for 1g on retainer, when you can get 1g for each item immediately at any vendor. But then I see stacks of 2-7g items for 1g, so, paying a tax on top of charging only 1g (price of stack will be more than 10 or 20g). They are getting <1g/item! And they could have gotten 2 or more gil at any vendor.

Anyway, the MB is fine. Prices since patch have plummeted because of rogues & ninja's doing LOTS of dungeons & quests. Thus huge supply, & sellers who round down a thousand = dropping prices. This is a buying opportunity. Go buy cheap & wait a bit, & when prices rise again, start selling these items back.