Is it permissible to sell a product online that differs in specifications and brand from another seller's product, but at the price of the product I have, until the website accepts my original product listing?

You are not permitted to sell an item that the website has rejected based on another seller offering it with a different description than yours. This is because the condition for selling by description is that the described item must match the description, and this constitutes deception and misrepresentation. If the item reaches the buyer and they discover the difference, they have the right to annul the sale, and the seller bears the transportation costs.

You are permitted to state explicitly that the item on offer differs in description from what you possess. This validates the sale according to some scholars who permit selling an absent item without description, in which case the buyer has the option of inspection. This is the view of the Hanafis, a narration from Ahmad, and preferred by Ibn Uthaymeen.

However, if the buyer sees the item and does not like it, they have the right to return it, and they cannot be compelled to keep it or bear the cost of its transportation back and forth. As for selling an item with a description that contradicts its true nature, it is a forbidden deception and misrepresentation, and it usually leads to disputes.