Does corresponding with the merchant and obtaining his approval before publishing his products on my website, with my own pictures and detailed descriptions, absolve me from the suspicion of selling what I do not own?

If you agree with a merchant to publish pictures of his goods on your website, and he authorizes you to sell them by setting a price for you, with your commission being whatever you add to his price, then there is no harm in that. In doing so, you are an agent for the owner of the merchandise in selling it. You inform the buyer of its price, and if he agrees, you send the agreed-upon price to the owner, who then sends the goods to the customer. Ibn Abbas said: "There is no harm in saying: 'Sell this garment, and whatever exceeds such and such is yours.'"