Double Ducks are a brand of straight razor that appear to be one of the most sought after collectable straight razors on eBay, LOL. Most common are the Goldedge, Wonderedge and Satinedge. I own a few and in the opinion of most including myself, they are excellent shavers but have been extremely overpriced lately. Heres a link to a Wonderedge that was up for auction.
Old dubl duck Wonderedge Straight Razor One of the Best - eBay (item 250685790568 end time Aug-29-10 19:00:55 PDT)
As far as what to look for in a Henckels. I would focus on the condition of the blade. Try to avoid rust, pitting and ugly overhoning that has eaten up the blade. I hear Henckels are great shavers too. I just got one but haven't had a chance to use it. I won it on an ebay bid. Pics were not that great so I didn't realize the toe( tip of blade) was honed down so much. This disappointed me but it still appears to be a usable razor. Here's a pic of it ( Razor on the left)