You can choose the price for each item you sell on ImpressPages Marketplace, but some limits for the lowest and the highest price might apply. Additionally you can choose how much of that price you want to contribute to ImpressPages. E.g. if you want to sell your theme for $100 and set the contribution to ImpressPages at the amount of 30%, then you will get $70 from each sale. All the money earned will be paid out to your PayPal account in two months. This time frame is required to make sure that sold items work well, and there are no disputes.
Formally, you are the seller of the product to the buyer. We just do the distribution service. According to the law in your country, it is your legal obligation to pay all taxes for the whole amount paid by the buyer.