Read Jim Kloetzel's article in published about ten years ago in Scott's Stamp Monthly magazine. He points out that many bidders bid low on
ebay out of fear that they will end up with "stamps they don't want and can't return". It is one of the reasons that better material doesn't sell for high enough prices at
ebay auction. The worst material sometimes sells for more than market value while the best material sells at a fraction of market value. Many sellers switched to fixed price to avoid dumping good material at low prices. Unfortunately, much of the junk has followed the better material to fixed price.
It is not uncommon for brick and mortar auction houses to sell collections with high catalog value at low realizations. Some of the collections are made up by dealers wishing to unload inferior stamps. Sometimes the lots are salted with a few better stamps in the hope that potential bidders will miss the mid-value junk mixed in. Remote bidder who bid low, lose out on almost all of the better lots and may end up buying the worst lots no matter how low they bid.
Some auction houses are better than others.
Clark