I have two serious winter bags for sale....and these are the Mercedes benz of sleeping bags. 

First, I have the mother of all Western Mountaineering bags, a Puma. And this is a tricked out Puma, at 6'6" length (so you can stuff things in the footbox that you don't want to freeze), with Goretex Windstopper material (gore dryloft), and best of all, this Puma has the Western Mountaineering overstuff option which adds 5 more ozs of 850+ fill power down. This easily pushes it to at least -30°. 

The Puma has a V-block side baffle, a full overstuffed neck down collar, and....incredibly, 2 overlapping full length zipper draft tubes (one on bottom and top). This particular puma is a right zip, and has the original storage sack and stuff sack. This bag is a 2012, but only used on one trip to Denali. Its in perfect condition, not a single stain or tear on it. Stored in a pet and smoke free home. 

Western Mountaineering doesn't allow retailers to put sales on their bags, so the only way to get one on sale is to buy used. A 6'6" Puma GWS with overstuff option is $1073 plus shipping retail. I am selling for over 25% off at $800 shipped free. UPDATE: $740

Next, I have a -10° Lynx GWS bag, 6' length. Also excellent condition, and also with the Goretex Windstopper material (gore dryloft as it used to be called). You want the beefier material for these winter bags. Mainly, b/c your tent generally snows from the inside in the winter, where your breath condenses and freezes on the inside of your tent, and when the wind blows, it shakes it off and "snows" on your bag. And, with the goretex windstopper material, you wouldn't ever need to use a bivy with this bag. 

This bag also has a full length zipper draft tube, side block baffle, and full down neck collar. Drafts will not get in! Just like the Puma, stored in a smoke and pet free home, and uncompressed in its storage sack. Comes with the original storage sack and stuff sack. 

A Lynx GWS retails for $870 plus shipping. I am selling for $640 shipped free, which is over 25% off! UPDATE: $595