My experience buying used down sleeping bags online has been very good. I have bought several different older bags online and have seldom had any issues with them after cleaning and fluffing them up. That said, there are several things to watch out for. You aren't going to get a Western Mountaineering bag cheap. You may get an older, decent name bag for a good price, say $30. Very few people will give you accurate weights and measurements, so you take some chances there. I don't think that this is from dishonesty as much as it is just simply not appreciating haw important these are to buyers. I have several bags that are just too small for me to use comfortably as a bag. They can often still be used as a quilt. I have a very nice Trailwise Slimline bag that is much too small for me, but is fine for my wife. I have a Gerry bag that I really like and a couple of heavier bags with great loft that I use for cold weather. As long as the bags are name brands that you can do some research on and you don't mind patching a hole, the risks aren't that bad. The down bags really do hold up much better than synthetic bags over the long term.