Does the e-marketing of products—without actually owning them and buying them later after the buyer has paid—fall under the contract of Salam, or selling what I do not possess?

The inquirer does not own the commodity at the time of the contract, and this type of sale is forbidden by Islamic law, unless it falls under the category of salam sale with its conditions, which are: handing over the capital at the contracting session, describing the commodity precisely in a way that removes any ambiguity, and that it be a debt (something owed) and not a specific existing item.

This does not apply to the inquirer's method of operation because the customer pays the price to "Shopify" company and not to the seller. This company is not an agent for the seller; rather, it is an intermediary. Moreover, the permissibility of agency in a salam contract is a matter of dispute among jurists.

There is another legal impediment, which is Shopify's combination of a monthly fee and a percentage of sales, which is impermissible according to the majority of scholars.