When is a sale considered a Salam (forward) sale, are the aforementioned cases of selling clothes online permissible, and what is the legitimate way for such trade?

The scenario mentioned in the question—the customer purchasing goods based on description—falls under the category of Bay' al-Salam (forward sale), due to the condition of specifying a delivery date for the commodity and the full payment of the price at the contracting session. This is achieved by transferring the price to the seller's account at the time of the contract.