Credits to Dan for Sales 1 - 4, spare the Minis.

http://backpack.tf/item/277920651

Sale 1) Sold for a Terror-Watt Caribbean Conqueror (14 Keys) and a Strange Specialized Killstreak Festive Black Box (~3 Keys)

http://backpack.tf/profiles/76561198044664114#!/compare/1467504000/1467590400

14 + 3 = 17 Keys

Caribbean Conqueror Mini

http://backpack.tf/profiles/76561198044664114#!/compare/1468022400/1468108800 - Sold for a Dead Head (14 Keys when the sale happened)

Sale 2) Sold + ~9.5 Keys of Items for a Circling TF Logo Barnstormer (26)

http://backpack.tf/profiles/76561198139560938#!/compare/1468281600/1468368000

26 - 9.5 = 16.5 Keys, which rounds to 17 Keys

Barnstormer Mini

http://backpack.tf/profiles/76561198299165673#!/compare/1467158400/1467244800 - Sold for a Terror-Watt Festivizer + MW Spec KS High Roller Medi (~4)

http://www.tf2outpost.com/trade/28299889 - Capped at 26 Keys

Sale 3) Sold for a Bubbling Buccaneer's Bicorne (12 Keys) and 4 Keys

http://backpack.tf/profiles/76561198115721685#!/compare/1468368000/1468454400

12 + 4 = 16 Keys

Buccaneer's Bicorne Mini

http://backpack.tf/profiles/76561198089034531#!/compare/1468195200/1468281600 - 12 Keys

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http://backpack.tf/item/894026423

Sale 4) Sold for 15 Keys

http://backpack.tf/profiles/76561198042938501#!/compare/1467676800/1467763200

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http://backpack.tf/item/280494207

Sale 5) Sold for a Duped Nuts and Bolts Private Eye + 3 Keys

http://backpack.tf/profiles/76561198117326723#!/compare/1465862400/1465948800

Somewhere in the range.

Sale 6) Sold for an Australium Stickybomb Launcher (15 Keys) + ~1 Key of Items

http://backpack.tf/profiles/76561198191570915#!/compare/1466121600/1466208000

15 + 1 = 16 Keys

Sale 7) Sold for a Steaming Hustler's Hallmark + 2 Keys

http://backpack.tf/profiles/76561198091312461#!/compare/1467763200/1467936000

Fits in the range.

Figured I didn't need to list more sales, as this is sufficient for a range of 15 - 17 Keys.