This is pretty clear cut:-

You are selling slicks at a profit (I am assuming you sell them often). Therefore you are trading ‘goods’! quote -
“but do sell cheap slicks for hardly any profit at all”

The fact you make very little / no profit is irrelevant, this just proves your not very good at business ;( . No one else cares how much profit you are making and it’s no ones business. At the end of the day you are trading, therefore you should have to pay traders fees.

Would you rent a shop and say to the estate agent, ‘Oh mate, I hardly make any profit, therefore I should pay a lower rent’??????

Also why should we take YOUR word you are making no profit. This could apply to every trader saying ‘ oh oh I’m not making any profit !!! so I don’t / won’t to pay fees’!!!

Sorry to sound harsh but it’s your problem if you’re a charity (PS im not a trader by the way !!)