I don't like this at all, for a few reasons:

1. Undercutting is a word that is being applied in a bad way. Undercutting is what happens when there's competition. If all sellers explicitly or implicitly agreed upon a common price this would price fixing. That is bad.

2. The listing fee you propose would only be a tax. Taxes are in turn passed to the consumer. They don't affect the seller. Hurting consumers is worse on the market than sellers since the padded price would discourage buyers, especially those buying goods they don't need.

3. It punishes sellers who don't have Gil to sell the product also. Sure that'll lower the number of sellers, which could increase selling prices, but remember most of these sellers have to buy goods to make their ware. If they decide that they'll incur more cost they may not buy or produce goods. That is bad for the market as a whole.

The issue of RMTs and bots is a valid issue. It's using the game in an unintended manner.