Relisting on Auctiva will work just like it does on eBay for receiving your relisting credit.
If you post an item and it does not sell, you can relist and receive the "relist credit" if that item does sell. This comes as a credit after the end of the auction, so you will initially be charged the listing fee's.
On the Auctiva site, on the closed listings page, there is a new filter so that you can display only the items that are "unsold". Look for the dropdown box labled "With Status" and then select the "Unsold" option.
-Kevin