You actually CAN use the "Best Offer" feature when listing through Auctiva. When you are creating a fixed price listings or store item, there will be a box beneath the "Buy it Now" field that states "Accept Offers from Buyers". Just check this box when creating your listings, and they will include the "Best Offer" feature.
I hope this information proves useful

http://www.auctiva.com/free_auction_software_help/help.aspx
-Mike