How many games? I'm assuming it's a large sample, because I've played 2 games that had this very same listing tax and the markets were as steady as could be expected in a real market. Supply spikes on weekends dropped prices, they'd rise again during the week.

Ffxiv as it is now can't have more problems with undercutting by large amounts than it already does. On several occasions I find markets that have a consistent price of (random) 6k. I list at 6k, 7 other people list at 6k; 12 sell per day. I come back in four hours and the price is 25%, 1500, of it's original 6k value; it stays there for two weeks.

I believe a listing tax can't possibly lead to worse undercutting, and to the comment about it being a rich person's market... yeahhhhhhhhh look around, it already is. Almost all video game markets are a rich person's market.