What is the ruling on a marketer taking a fee for marketing a product by increasing its original price to the buyer, with the marketer bearing the costs of advertising, and the buyer only knowing the final price?

There is no problem with increasing the price for the sake of profit, whether the advertisements are paid or free. However, if your work is limited to marketing without purchasing the product and reselling it, then agree with the product owner on a known commission or fee to be added to the selling price for each product. The buyer is not required to know the details of this price.