Hi,
Your listings will only disappear once, its to append the scrolling gallery to all your current auctions.
Its ebay who's holding up the relisting, while they recheck and relist and reenter them into search etc.
Any future autions you do will automatically add the scrolling gallery on time of creation.
It doesn't matter whether you list through ebay or auctiva, but if you list through ebay you won't get the added benifits of auctiva (free supersize, free photos, free templates etc)