This is a list of preferred things for the first photograph (or picture) to advertise a book on ebay.
1/ The front cover, if it is not plain or damaged. (Of course if it is damaged you should still take a picture of it as a second picture, or mention it at least).
2/ The illustration at the front-that is, the frontispiece.
3/ An illustration in the book which is relevant to the story.
4/ An illustration which isn't relevant to the story.
5/ A photograph of the author or authors.
6/ The title page of the book.
7/ The front cover if it is plain.
As you can see, showing a plain front cover will not make people buy a book. It could be any book. Something more colourful is much more likely to sell.