The commission paid to the gallery is a selling expense that reduces the sales proceeds reported for the transaction. Thus, the commission effectively reduces the amount of gain (or increases the amount of loss) for the sale.
Advance payment of current tax liability based either on wage withholdings or installment payments of your estimated tax liability. To avoid penalties, you generally must pay to the IRS either 90% of your final tax liability, or either 100% or 110% of the prior year’s tax liability, depending on your adjusted gross income.