Bidorbuy ignorance

I listed an item on Bidorbuy (and other sites) and I sold the item on another site. Before I had the chance to remove it from Bidorbuy, the item sold there as well. The buyer did however contact me and I informed her that it was sold, therefore she never paid me. I then attempted to remove the listing, but couldn't, as the item was 'sold'. I then got an invoice from Bidorbuy for the 'sold' item and no matter how I explained my position, they still expect me to pay sellers fees, even though the item was not sold or paid for through Bidorbuy. There policy is that where a Seller cannot supply the advertised item due to the item not being in stock, they will not credit the fees. I am not a shop and do not have 'stock'. This was a once off sale and no matter how i explain it, they will not listen. This is ******* and like being charged Vat on an item you were browsing in the shops but never bought. I will not be using Bidorbuy again.