There is currently a bug on the market when the price of the highest buy order and the price of the lowest sell order are exactly the same. When you try to sell an item in this case, the system will create a sell order with that price, instead of selling it.

While we fix this, the workaround is to manually place a sell order where you set a price slightly lower than the highest buy order's price. The item will be then sold automatically to that order.