I personally don't understand why we (the seller) are taxed AFTER the transaction is made.
The first time I went to sell something, I expected to be charged a couple percentage for the privledge of posting the item on the boards. I was pretty surpirsed when I wasn't charged the fee until after the sale occured.
In short, I agree with the OP.
Charge a "posting" or "listing" fee, and then charge an additional fee for re-posting of the same item at another rate.
It would do a couple of things:
1) It would most likely put a stop to the majoirty of weirdos who put not very rare items up for hundreds of thousands to millions of gil (when there are dozens of lower prices already on the board)
2) It would stop the rampant undercutting from people who religiously live off of the market board
NOTE: I'd also be fine with SE implementing a caveat into the system in which, if your item goes a full week and no on purchases it... it's removed from the board (to your retainters inventory), and you're reimbursed your fee in full. You can then decide to re-post the item or not.
Example:
I have a helm. I put it up on the market board for 1,000. I'm charged a fee of 50 gil for doing this.
I log on tomorrow morning and see that there are now 50 of the same type of helm on the board, and 20 of those helms are priced lower than mine.
I can either... take the helm down (forfit the 50 gil that I already paid), and re-post it for 500 gil, and eat a fee of 25 for doing this.
Or, I can just wait a week, hope it sells... if it does not, at the end of the week the item is returned to me along with the 50 gil in costs. At that point I can go ahead and re-put it up on the board for 500 gil (and be charged 25), or I can just keep the item and not be out the 50 gil in fees.
I don't know that this solves EVERY problem, but I do think it puts a dent in the weird temporary deflation spikes that individual items sometimes see. It also removes a lot of clutter at the other end of the board (who is going to pay 30,000 gil for a lvl 35 helm, when there are 49 of the same helm going for 5% of your list price? Answer, no one... you're wasting space on the board)