I am not a civil war collector or an ebay buyer. So I cant say if it looks authentic or not, especially from a photo.
many years ago bought off ebay, and had an ok experience. its always buyer beware.
On the plus side this seller belongs to many organizations as shown on his profile:
Member: American Society of Arms Collectors, Company of Military Historians, New England Antique Arms Society, Forks of the Delaware Historic Arms Society, Ohio Valley Military Society. Nationally recognized Antique Arms & Militaria Consultant.
And he has a wide assortment of other buttons as shown in his online store, 477 currently listed.
He states "All buttons are guaranteed to be original and of the period indicated in the description. "
The description being "excellent example of a 2 piece Civil War period Confederate Army General Service "C S A" button."
With no negative feedback it may be a decent opportunity if the price is right for you and you feel the item for the price is equitable.