Now that would be cool. If eBay let you enter both a full access password, and a bid only password. Then you could give that password for bidding to places like AuctionSniper.
I think the odds of eBay adding that feature is probably about a zillion to one. It took the like 4 years to add the ability to add pictures to your auction when listing your item with them. And that's a pretty darn key feature.