From what I have been told by Holts the seller fee is 15% on first £1000 and 10% on the remainder all +VAT, that's negotiable on how much you are selling, what you are selling and so on.
So as the gun sold for £3300 the fee would be (150+230)*1.2= £456, so the seller would actually get £2844, the buyer actually paid £4290 inclusive of 25% buyers fee and VAT on the fee.
I make that over £1000 of profit in this case...