Hi shoes,
Auctiva does not currently support posting eBay Giving Works Charity auctions. However, we do plan on adding this feature to our site with our next big release, which is tentatively scheduled for February 2007.
In the mean time, if you have already created a saved listing for the item you would like to list, you can retrieve the descirption html for that listing using the "Get HTML" button on the Saved Listings page.
Then, when you create your charity auction using eBay's "Sell your Item" form, just paste the html code generated by the "Get HTML" button into the html version of the description editor, and complete the rest of the listing as normal. This will cause the listing to look like the one you have designed in Auctiva.
-Mike