What is the ruling on selling a product I do not own, by agreement with its owner, such that I display it online, then send the buyer's information to the original seller for him to ship the product and collect the price, after which the original seller gives me my agreed-upon profit?

There is no objection to displaying a product online and selling it in this manner as an agent for its owner. However, it is not permissible to take a profit exceeding what the product owner has specified or what was agreed upon. Any increase in price after the agreement belongs to the principal (the product owner), unless they have authorized you to keep it.