Auctiva does not get a commission from eBay, Mel.
As said before, they get a cut of the sales from Auctiva insurance. They also have a paid version of their service which they call Classic Auctiva. If you scroll down to the bottom of the main Community forums page, you will see 3 forums dedicated to Classic Auctiva.
Also, they now own Sellathon, so the make money off of the monthly subscription fees from there.