Welcome to tPF.
The bag in the listing is authentic.
While I understand your concern about sellers not taking returns, I can explain the reason they say it.
Though many sellers consider selling on ebay to be a business, others do it more as a hobby and would prefer not to be treated like Macy's or
Nordies. Accepting returns is time consuming, expensive and would force sellers to raise their prices to cover the additional time and expenses.
Generally many sellers advertise "no returns" to avoid hassles. When the seller makes a mistake in her listing, most good sellers are willing to accept a return due to their mistake.
And even when the return isn't the result of seller error, many sellers don't want an unhappy customer and will allow a return as long as the buyer is honest about the reason for return, item is in its original condition, returned in a timely manner and shipping is paid by the buyer.
If you are concerned about it, you can pose a question to the seller offering to cover shipping in both directions should the item not work for you.