I'd love to hear how changing the post sale tax on the seller to a pre-sale list fee of the same amount would not help balance things out a bit. Saying something like this doesn't really mean anything without giving a bit of explanation. Quite frankly, nothing anyone against the OP's suggestion has said has made any good, logical point on why it's a bad idea, beyond "that doesn't sound good, I don't like it." Some people may be thinking they would be getting double charged to sell. That's not how it would work. It's not how it worked in FFXI and I would bet its not how it worked in WoW. It would and should be just 1 charge, but up front.
Would it fix the problem completely? Of course not. People with lots of cash will always be able to buy up all of a particular item up for sale to corner a market (which brings RMT into the discussion). But the vast majority of players will put items up for sale at a price, maybe undercut a bit, maybe try to get a bit more if supply is low. But if they are paying 5k to list a 100k item, they aren't going to sit at the Market Board, constantly checking to see if someone put up the item for a bit cheaper. People would be more inclined to have a bit of patience to wait those 7 days items stay for sale on your retainer. Maybe you decrease the time they stay up. I think it was 3 or 4 days in FFXI. Heck, you could refund the listing fee if the seller waits the full amount of time rather than taking it down prematurely. That seems pretty fair to me, actually. But this allows the vast majority of the market (see above about very rich players and RMT) to operate more on supply and demand.
And to anyone arguing that's not how it works in real life. First of all, in retail, yes it is. A distributor that wants to sell a food item at a Walmart or a Target or whatever has to pay a "slot fee" to get their product a spot on the shelving. Second of all, even in markets where this doesn't apply, I argue you can't compare the game economy to real life. Mostly just because people obviously care much much more about their real life money than virtual cash in an mmo. People aren't willing to totally hamstring themselves financially to make a quick sale in real life. You don't see two gas stations on opposite corners of each other constantly slashing their gas prices to make sure they have the lower price. They understand thats not good for business. They usually have the exact same price. In an MMO, a vast majority don' have that business sense. Many simply don't care too much. Especially in a game where in game currency doesn't get you much beyond purely vanity items like an expensive minion or housing. But thats another discussion. Many may value inventory space more than a few more gil, so they sell stuff super cheap to get rid of stuff. The point being that the economically uneducated and apathetic need some help and limits from above to keep the economy healty