When there is one seller, it could mean they are the exclusive seller for the brand. This means they have the right to sell the product, but you as a seller may not (even if you are ungated). Amazon may allow you to list the product, but the brand itself has the power to kick you off the listing.
Whenever you see one seller on the listing, check Keepa to see if there are usually only 1 or 2 sellers, or if there have often been a lot more in the past. This gives you a better idea if the one seller is an exclusive seller or just happens to be the only seller at the time you are looking at the product.
You would want to confirm with your supplier (and possibly ask for a letter of authorization from the brand is you are not confident) or reach out to the brand themselves to double-check that you're able to sell alongside the other seller.
Also, whenever there is one seller, a lot of the time it is the brand itself. Make sure you check who the seller is and see if they are the brand. If so, then there's a good chance they are the exclusive seller and you would want to get in touch with them to make sure you can sell alongside them.