Re-suggesting with a new range as advised by Marty.

PK Freeze

http://backpack.tf/item/1680319476

Sale 1) Sold for a Specialized Killstreak Australium Grenade Launcher (~14 Keys)

http://backpack.tf/profiles/76561198116572221#!/compare/1494115200/1494201600

Low Outlier

Sale 2) Sold + 1 Key for a Max's Severed Head (19 Keys)

http://backpack.tf/profiles/76561198231097008#!/compare/1494374400/1494460800 - Buyer's Backpack

19 - 1 = 18 Keys

http://backpack.tf/item/719272055

Sale 3) Sold + Schadenfreude (~1 Key) for a Blizzardy Storm Samur-Eye (16 Keys)

http://backpack.tf/profiles/76561198067771676#!/compare/1492128000/1492646400

Low Outlier

The following is credited to Marty.

Sale 4) Sold for 19 Keys

http://backpack.tf/profiles/76561198183681006#!/compare/1495584000/1497139200

Since 18 and 19 were both pure, I'll go 18 - 19