I agree that it would probably be helpful if it clarified that the 'house cut' is actually only 1/2 of that and the other 1/2 is a fee which disappears. That way traders will have a better understanding of how much their guild is getting from sales.

I don't think there's any problem with it happening however, because games need gold sinks to balance out the gold which constantly enters the economy from drops and items sold to merchants, otherwise you get rampant inflation. I remember playing Ultima Online before they figured that out (I assume they did fix it at some point) and it just got silly, even basic items purchased from other players cost a small fortune, which made it difficult for new or casual players to afford anything. Even for long-term players it was a pain because you had to fit actual piles of coins into your bags.

Putting a gold sink on trading, as a percentage of the item value is pretty effective because it means how much you pay is based on the value of the items you're selling and therefore roughly proportional to what you can afford. No one will be left out because they can't afford the fees, but it constantly drains gold from the economy.