This will be tricky with the way Auctiva has the "preview" set up but it can still be done.
Here's what to do:
Make your listing just like you would for ebay, then preview the listing, then right click to "view source" a new window pops up giving you the HTML for the listing.
Now the tricky part is removing the ebay stuff that Auctiva has in there but once you do it a few times you get to know where your own listing starts and stops fairly easy.
Take that HTML over to yahoo, bidville, TAM or wherever and past the it over there and your listing is done.
I know it sounds like a PITA but its actually really easy once you do it a few times