I'd love to see a tool that would allow me to post a gallery, like the sliding gallery utility of selected [not just all] auctions to post on other websites and communities. There is a large community site for handmade items that does this currently and it's very effective for attracting buyers.
I'd also like to see more tasteful options for promoting auctiva within my auctions. 3 seperate divs of link ads appended to each auction is a little excessive and I think you'd gain more new, especially younger, sellers with less in-your-face advertising. Just my opinion. Less is sometimes more.
Also, the generic template styles are terribly out of date and could use a revamp. Obviously eliminating old styles could upset the users that have been using them for years and we wouldn't want to do that. If it were my project, I'd move them all to a separate section and redo the entire template system. Topical, clip-art style graphics are pretty much graphic design suicide in this day and age. Simple color and style based templates would gain you a lot of respectability, and probably a lot more members, and wouldn't take all that much time to create.