Haha, I'm an idiot.

https://backpack.tf/item/6299075396

Sale 1) Sold for 19 Keys

https://backpack.tf/profiles/76561198111369293#!/compare/1521417600/1521504000

https://backpack.tf/item/4284021361

Sale 2) Sold for 17 Keys + 19 Refined

https://backpack.tf/profiles/76561198165294687#!/compare/1519603200/1519862400

Low Outlier.

Sale 3) Sold for 20 Keys + 7 Refined

https://backpack.tf/profiles/76561198337734785#!/compare/1527120000/1527206400

Rounds to 20 Keys

https://backpack.tf/item/4312590591

Sale 4) Sold for 18 Keys

https://backpack.tf/profiles/76561198264702468#!/compare/1521244800/1521331200 - Buyer's Backpack

Low Outlier.

https://backpack.tf/item/4312813344

Sale 5) Sold for a Starstorm Slumber Nasty Norsemann + Victory Lap (2.75 Keys)

https://backpack.tf/profiles/76561198002757046#!/compare/1519862400/1519948800

Capped by the Buyout of 20 Keys (See Sale 11)

Sale 6) Sold for 22 Keys

https://backpack.tf/profiles/76561198060967458#!/compare/1520726400/1521158400

https://backpack.tf/item/4372245112

Sale 7) Sold + 10 Keys + 16 Refined (~0.6 Keys) for a Blizzardy Storm Dr's Dapper Topper (33 Keys)

https://backpack.tf/profiles/76561198146822111#!/compare/1520640000/1520726400

33 - 10.6 = 22.4, which rounds to 22 Keys

Sale 8) Bulk sale for a Tesla Coil Executioner

https://backpack.tf/profiles/76561198302706987#!/compare/1521158400/1521244800

The Billycock wasn't involved. A Smoking Pillar was: https://backpack.tf/item/1875405257

Sale 9) Bulk sale for a Morning Glory Defiant Spartan

https://backpack.tf/profiles/76561198064690893#!/compare/1521158400/1521244800

Sale 10) Sold + Specialized Killstreak Australium SMG (~7 Keys) for a Haunted Ghosts Desert Marauder (32 Keys)

https://backpack.tf/profiles/76561198261840395#!/compare/1523750400/1523836800

32 - 7 = 25 Keys

Desert Mini

https://backpack.tf/suggestion/5b08ca6844325a71ea1ba2b9 - 32 Keys

https://backpack.tf/item/4993690475

Sale 11) Sold for 20 Keys

https://backpack.tf/profiles/76561198002757046#!/compare/1520121600/1520208000

Sale 12) Sold for a Professional Killstreak Australium Grenade Launcher (~23 Keys)

https://backpack.tf/profiles/76561198051395926#!/compare/1525564800/1525651200

https://backpack.tf/item/5959252457

Sale 13) Bulk sale

https://backpack.tf/profiles/76561197997810328#!/compare/1521417600/1521504000

Sale 14) Sold + 2 Keys for a Haunted Ghosts Engineer's Cap (26 Keys)

https://backpack.tf/profiles/76561197997810328#!/compare/1521849600/1522022400

26 - 2 = 24 Keys

Engineer's Cap Mini

https://backpack.tf/suggestion/5b08f72ccf6c75175518dadf - 26 Keys

Sale 15) Sold for a Terror-Watt Powdered Practitioner + ~6 Keys of Items

https://backpack.tf/profiles/76561198052218255#!/compare/1522022400/1522108800

Fits in the range.

Powdered Mini

https://imgur.com/a/vfUJZrs - 2 Sellers at 17 Keys and 1 at 20 Keys suggests the Powdered is <20 Keys.

Sale 16) Bulk sale

https://backpack.tf/profiles/76561198052218255#!/compare/1522368000/1522454400

Sale 17) Sold for 21 Keys

https://backpack.tf/profiles/76561198056793444#!/compare/1522713600/1522800000

https://backpack.tf/profiles/76561198197255435#!/compare/1522713600/1522886400

https://backpack.tf/item/6039279773

Sale 18) Sold for 19 Keys

https://backpack.tf/profiles/76561198002757046#!/compare/1520035200/1520121600

Sale 19) Sold for 21 Keys + 23 Refined

https://backpack.tf/profiles/76561198304138933#!/compare/1524873600/1524960000

https://gyazo.com/bd18c5998e28576d87e2e43cdfc2f2bf

Rounds to 22 Keys. Credits to Erik.

https://backpack.tf/item/6323802493

Sale 20) Sold for an It's a Mystery to Me Hellmet

https://backpack.tf/profiles/76561198310088025#!/compare/1526515200/1526601600

High Outlier, as the Hellmet has a Buyer at 25 Keys: https://imgur.com/a/rWZJxqk

https://backpack.tf/u/76561198002757046 - The Hellmet Buyer

https://imgur.com/a/Fcw1J1N - Using these 5 Buyers at 18 Keys and up to rule 18 Keys too low.

https://backpack.tf/u/76561198032176942

https://backpack.tf/u/76561198358337981

https://backpack.tf/u/76561198337734785

https://backpack.tf/u/76561198304138933

https://backpack.tf/u/76561198002757046