I have no issue with the tax being there. My issue is that the playerbase doesn't understand it. Buyers and sellers alike.

The MB should have the tax built-in to the listing price. Therefore, let's say this is the MB listing for an item:

I check in Gridania:
Item A - 104,000 (seller in Gridania, no tax)
Item B - 105,000 (seller in Limsa, tax)

I check in Limsa:
Item B - 100,000 (seller in Limsa, no tax)
Item A - 109,200 (sell in Gridania, tax)

Listing change based on where you are when you check them. Ask anyone who undercuts... why did you do it? "So my item will be listed first." Well, you could three different sellers listed first if the tax was built into the list price on the MB. And worst case scenario (bad for sellers, good for buyers) is that sellers would just undercut each other into oblivion to beat that 5% tax and be listed first everywhere. However, at least they wouldn't do this 1 gil undercut business.