Would love to have provided more proof, but quite frankly, there isn't any. Only half a page of these on OP, most of them as part of a set. Still, it seems clear from looking at OP that these are very hard to sell at 5 keys, and some person has steadily decreased his price until his most recent trade, for 4 keys.

Unsold for 5 keys

http://www.tf2outpost.com/trade/7789316 - Unsold for 5 keys

http://www.tf2outpost.com/trade/7666995 - Unsold for 5 keys

Selling for 4 keys

http://www.tf2outpost.com/trade/7815659 - Selling for 4 keys

Buying for 4 keys

http://www.tf2outpost.com/trade/7810726 - Buying for 4 keys

All that I can give you, unfortunately. Let me know what you think :D