2 years is a lot of time...

Sale 1: http://backpack.tf/profiles/76561198192182682#!/compare/1494115200/1494288000

Sold for a Clean Circling Heart Charmer's Chapeau (77 keys)

B/O at 70: http://www.tf2outpost.com/trade/28829253 but sold pretty fast for 77 in unusuals.

Sale 2: http://backpack.tf/profiles/76561198192182682#!/compare/1494460800/1494633600

Sold with Burly Beast (3 keys) for a Clean Death at Dusk Combustible Kabuto (71 keys)

71 - 3 = 68

Sale 3: http://backpack.tf/profiles/76561198138611987#!/compare/1495929600/1496188800

Sold for a Clean Molten Mallard Nunhood (72 keys) + 8 pure

72 + 8 = 80

B/O at 84: http://www.tf2outpost.com/trade/29195769

Sale 4: http://backpack.tf/profiles/76561198192182682#!/compare/1496102400/1496275200

Sold with Duck Billed Hatypus (1 key will be fine anyway) for a Clean Death at Dusk Combustible Kabuto (71 keys)

71 - 1 = 70

Sales: 77, 70, 80 and 70. A range of 70-80 should work perfectly.