Thank you for your question on importing products from ebay to PrestaShop. Currently, the only solution we know of is via a paid module. You can view it here. We are not aware of a free version currently and know that it is not a default feature.
I did some checking and cannot find a tool that allows you to export from Auctiva and import to PrestaShop. The only thing close that I found were Freelance services. I'm fairly sure they charge a decent premium to do that.
It might be that you would have to export to CSV from Auctiva (if Auctiva can do this). Then you could take the CSV file and import it into PrestaShop. Not a direct export, but it still hopefully gives you an option.