Is it permissible to sell in this manner, with an increase in price without the buyer's knowledge? What is the ruling on dealing with another marketer in the profit sharing? Is there a specific amount for the added commission? And what should be done regarding previous sales transactions that occurred without the buyer's knowledge of the added commission?

It is permissible to market goods in general, unless it involves deception or the good is prohibited.

As for the commission, it can be a fixed amount, or whatever exceeds the price requested by the owner of the good. It is not a condition to inform the buyer of this, for Ibn Abi Shaybah narrated from Ibn Abbas that he saw no harm in a man giving another man a garment and saying: "Sell it for such and such, and whatever you get extra is yours."

Similarly, marketing a good to another marketer who does not own the good, in exchange for sharing the commission upon sale, is permissible. This falls under the category of brokers collaborating.