I've had the same issue..
The issue is, is that on your ebay preferences when the box is ticked which says something to the effect of 'do not allow bidders without a paypal account' that all listings posted through auctiva should have this option reflected once it is submitted into ebay.
There is a bug however which does not recognize this setting when the listings are sent from auctiva through to ebay (at least this happens to me as well as the above posters).
I sent in a request awhile ago but I don't think anything can be done with the current version of Auctiva.
So as far as Auctiva listings SHOULD go, the option of not allowing NON paypal mebers to bid is a setting in ebay (not Auctiva) and ebay is supposed to apply that to listings received from auctiva which it doesnt for whatever reason.