Thanks for the additional sale, mastikhor.

https://backpack.tf/item/2466346022

Sale 1) Bulk sale for a Purple Energy Conjurer's Cowl

https://backpack.tf/profiles/76561198038878439#!/compare/1545264000/1545436800

https://backpack.tf/item/2622672360

Sale 2) Sold for 21 Keys + 13 Refined to a buy order

https://backpack.tf/profiles/76561198052455517#!/compare/1540339200/1541116800

https://cdn.discordapp.com/attachments/449219675394539521/536128018054381578/Screenshot_20190119-102034_1.jpg - Thanks, Eclips

Rounds to 21 Keys

Sale 3) Sold for 25 Keys

https://backpack.tf/profiles/76561198317606171#!/compare/1541462400/1541548800

https://backpack.tf/item/2726132257

Sale 4) Sold for an Open Mind Highway Star

https://backpack.tf/profiles/76561198119634686#!/compare/1542758400/1542844800

https://backpack.tf/profiles/76561198357301658#!/compare/1542412800/1542844800

This would be better used to price the Highway Star.

Sale 5) Sold for a Disco Beat Down Private Eye

https://backpack.tf/profiles/76561198357301658#!/compare/1542844800/1543968000

https://backpack.tf/profiles/76561198153417214#!/compare/1543017600/1543104000

This is being used for the Private Eye.

Sale 6) Quicksold for 21 Keys

https://backpack.tf/profiles/76561198153417214#!/compare/1543017600/1543104000

Sale 7) Sold for 22 Keys

https://backpack.tf/profiles/76561198073438708#!/compare/1546214400/1546300800

https://backpack.tf/item/2024479702

Sale 8) Sold + Tesla Coil Cranial Carcharodon for 33 Keys

https://backpack.tf/profiles/76561198088826566#!/compare/1540857600/1540944000

https://backpack.tf/profiles/76561198132844160#!/compare/1540857600/1540944000

Bulk sale.

Sale 9) Quicksold for 21 Keys

https://backpack.tf/profiles/76561198132844160#!/compare/1540944000/1541030400

Sale 10) Sold for $49.99 when Keys were ~$1.68 each

https://marketplace.tf/items/108;5;u62

https://marketplace.tf/items/5021;6/Mann%20Co.%20Supply%20Crate%20Key

49.99 / 1.68 = 29.76, which rounds to 30 Keys

High outlier.

https://backpack.tf/item/5054568683

Sale 11) Sold for 21 Keys + 13 Refined to a buy order

https://cdn.discordapp.com/attachments/449219675394539521/536128352378159104/Screenshot_20190119-102213_1.jpg - Thanks, Eclips

Rounds to 21 Keys

Sale 12) Sold for 22 Keys

https://backpack.tf/profiles/76561198317606171#!/compare/1545696000/1545782400

Sale 13) Sold for 21 Keys + 10 Refined

https://backpack.tf/profiles/76561198119634686#!/compare/1546473600/1546560000

https://backpack.tf/profiles/76561198251214662#!/compare/1546473600/1546560000

Rounds to 21 Keys.

Sale 14) Sold for a Terror-Watt Hot Dogger

https://backpack.tf/profiles/76561198251214662#!/compare/1546560000/1546646400

https://backpack.tf/profiles/76561198127938353#!/compare/1546560000/1546646400

Low outlier.

Dogger Mini

https://backpack.tf/profiles/76561198011321852#!/compare/1546646400/1546732800 - Sold for a Haunted Ghosts Milkman

https://marketplace.tf/items/614;5;u57 - Sold for $50 when Keys were ~$1.73 each, so this sale is ~29 Keys

Milkman Micro

https://backpack.tf/profiles/76561198232041512#!/compare/1546732800/1546819200 - Sold for 20 Keys

https://backpack.tf/profiles/76561198313538268#!/compare/1547337600/1547424000 - Supposedly was 23 Keys

https://imgur.com/a/gZttsJk - Bot.tf showing the sale was supposedly 23 Keys.

Sale 15) Quicksold for 22 Keys

https://backpack.tf/profiles/76561198127938353#!/compare/1546560000/1546646400

Sale 16) Sold + 2 Keys for an Orbiting Fire Charmer's Chapeau

https://backpack.tf/profiles/76561198418083011#!/compare/1546646400/1546732800

https://backpack.tf/profiles/76561198114402400#!/compare/1546646400/1546732800

I asked NervousPyro, and he said that 2 Keys were added with the Billycock -> https://imgur.com/a/D3LysFW

Low outlier.

Sale 17) Sold for 25 Keys

https://backpack.tf/profiles/76561198114402400#!/compare/1547078400/1547164800

Sale 18) Sold for 26 Keys

https://gyazo.com/9281bc81021182e4eba41a795002f0de - Thanks, mastikhor

https://marketplace.tf/items/108;5;u62 - Sold for $35.54 when Keys were ~$1.76 each

https://marketplace.tf/items/5021;6/Mann%20Co.%20Supply%20Crate%20Key

35.54 / 1.76 = 20.19 Keys, which rounds to 20 Keys

Calling 21 Keys low since most of the sales were quicksales or sales to buy orders.