Need some ideas and input.

[Bottle4]

I suggest the fee is taken after the item has been sold - it would be dumb if you put an item that no one will even buy AND pay up a fee in the same time - immediate money lost. Instead, how about it takes a percentage of the whole purchase? Like the the buyer pays 100% of the price, but the seller gets 70%-90% of the coins, since 10%-30% of the price goes into taxes, so you'll surely be on win - because if the item doesn't get sold, you don't lose anything.
And this is directed not only to the auctions - this is directed to the whole market, especially those with the firm. Prices.