150-200 gralats is reasonable. You wouldn't pay 200 gralats to be sure you aren't scammed?
The trades will be logged and therefore it isn't ideal to have the logs spammed with useless trades like 1 flower for 1 shell etc. The fee reduces the amount of players trading things not worthwhile. There will be a limited amount of tables as well, so adding a reasonable fee would prevent players from hogging the tables by trading flowers back and forth all day.