Hi Max,
Actually, relisting through Auctiva is essentially creating a new listing so there's no decision to be made between relisting and creating a new one. To relist something through Auctiva, just go to your saved listings page, click the boxes next to the items you want to relist, and click post or schedule. These relists will have different item #'s than the original listings.
If you relist through eBay, it typically takes 4-6 hours for the listings to be imported into Auctiva. Items relisted through eBay, however, will not go through Auctiva checkout. The buyer will still be able to check out smoothly through eBay checkout though.
I hope this clears things up=)
-Mike