Each seller on Black Market should pay a selling fee which is applied when someone purchases the item.
If you sell your item for gold, a fee of 10% will be deducted when someone purchases the item. A minimum fee is 1 gold coin, so you should set your price above this value or you won't receive any gold.
If you sell your item for gems, a fee of 10% will be deducted when someone purchases the item. A minimum fee is 1 gem, so you should set your price above this value or you won't receive any gems. In case you set a price equal to 1 gem, you won't receive any gems, so you may want to set a minimum price of 2 gems.