Excluded all sales over a month old.

Sale 1:

Sold for 21 keys

Seller: https://backpack.tf/profiles/76561198162282453#!/compare/1520726400/1520812800

Buyer: https://backpack.tf/profiles/76561198049017075#!/compare/1520812800/1520899200

X = 21

Sale 2:

Sold for $34 on Marketplace.tf

https://marketplace.tf/items/30816;5;u3005

https://backpack.tf/item/6300086975 (user is banned on alt for trading with scammers, so it should still be fine to use their sale here.)

34/1.97 = ~17.3

X = 17

Sale 3:

Sold for Pro. KS Aus Tomislav (~17) + anger

No outpost

Seller: https://backpack.tf/profiles/76561197994697258#!/compare/1520899200/1520985600

Buyer: https://backpack.tf/profiles/76561198308245489#!/compare/1520899200/1520985600

X = 17

Sale 4:

Sold for 15 keys + 2 ref

Seller: https://backpack.tf/profiles/76561198039912258#!/compare/1520985600/1521072000

Buyer: https://backpack.tf/profiles/76561198126679929#!/compare/1520985600/1521072000

X = 15

Sale 5:

Sold to a quickbuyer for 8 keys and some ref

Seller: https://backpack.tf/profiles/76561198086747482#!/compare/1519776000/1519862400

Buyer: https://backpack.tf/profiles/76561197994697258#!/compare/1519776000/1519862400

X = 8

Sale 6:

Sold for Strange Galactic Codex Universal Translator + Uncraft Voodoo JuJu (2)

http://www.tf2outpost.com/trade/29580363 - B/O of 20

Seller: https://backpack.tf/profiles/76561198004610923#!/compare/1520553600/1520726400

Buyer: https://backpack.tf/profiles/76561198065176679#!/compare/1520640000/1520726400

Easily fits in the range.

Sale 7:

Backpack buy

Seller: https://backpack.tf/profiles/76561198442748136#!/compare/1520553600/1520640000

Buyer: https://backpack.tf/profiles/76561197961172123#!/compare/1520553600/1522281600

Sale 8:

Bulk for Insomnia Coldsnap Cap

Seller: https://backpack.tf/profiles/76561198267583305#!/compare/1520640000/1520726400

Buyer: https://backpack.tf/profiles/76561198162282453#!/compare/1520640000/1520726400

Sellers:

15 keys:

http://www.tf2outpost.com/trade/29576337

20 keys:

https://i.imgur.com/iKCCrV0.png

https://backpack.tf/item/5275676682

22 keys:

https://i.imgur.com/mRHkHZd.png

https://backpack.tf/item/5275676682

23 keys:

https://i.imgur.com/s5Mqz1n.png

https://backpack.tf/item/5185061451

38 keys:

https://i.imgur.com/o5IVR9W.png

https://backpack.tf/item/5199819114

89 keys:

https://i.imgur.com/zC6qKrQ.png

https://backpack.tf/item/5843163736

https://backpack.tf/item/5185343458

Offers:

http://www.tf2outpost.com/trade/29592154

http://www.tf2outpost.com/trade/29528940

SCM:

https://i.imgur.com/pTrhlYg.png

$35.74 (~14.4 keys)

$40.63 (~16.3 keys)

$64.86 (~26 keys)

$270.86 (an unrealistic amount of keys

Sales at:

8

15

17 (2x)

21

Gonna ignore the single seller at ~14.5 keys on the SCM, as I can’t tell how long it has been there for.

8 went to a quickbuyer, excluding 8.

Going with 15-21.