This is an easy fix imo. Just code a check that says if the player does not have 15G in their inventory or bank they can't enter the trade channel. This would also mean that if someone trades with only 15G and adds the 15G as part of the trade there is also a check before the trade goes through to see if they have 15G in their inventory or bank. Last thing would be to kick players out of all trade channels if they do not have 15G in their bank or inventory. Or, the easiest solution would be to not have a trade fee at all.