Only using the last 2 months

Sale 1:

Sold for 24 keys + 20 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1524614400/1524700800

Buyer: https://backpack.tf/profiles/76561198134932459#!/compare/1524355200/1524700800

Bot.tf confirms: https://imgur.com/ZWLNLZk

X = 25

Sale 2:

Sold for 23 keys + 25 ref

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1523836800/1523923200

Buyer: https://backpack.tf/profiles/76561198304138933#!/compare/1523836800/1523923200

Confirmed by Bot.tf: https://i.imgur.com/ZWLNLZk.png

X = 24

Sale 3:

Sold for 23 keys + 28 ref

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198309838525#!/compare/1523059200/1523145600

Bot.tf confirms: https://i.imgur.com/ZWLNLZk.png

X = 24

Sale 4:

Sold for 24 keys

Seller: https://backpack.tf/profiles/76561198337734785#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198011234844#!/compare/1523145600/1523232000

X = 24

Sale 5:

Sold with ~2 keys in items for Australium Medi gun (~24)

Seller: https://backpack.tf/profiles/76561198071983249#!/compare/1522281600/1522368000

Buyer: https://backpack.tf/profiles/76561198079468968#!/compare/1522022400/1522368000

X = 22

Sale 6:

Sold with ~3 keys for Dead Presidents Nunhood

Seller: https://backpack.tf/profiles/76561198309838525#!/compare/1523318400/1523404800

Buyer: https://backpack.tf/profiles/76561198153482617#!/compare/1523318400/1523404800

Can probably be used on the Nunhood.

Sale 7:

Sold for Strange Fountain of Delight Burstchester

Seller: https://backpack.tf/profiles/76561198153482617#!/compare/1523491200/1523577600

Buyer: https://backpack.tf/profiles/76561198065733542#!/compare/1523404800/1523491200

Not using a taunt sale here.

Sale 8:

Sold for Disco Beat Down Little Buddy + 13 keys

No outpost

Seller: https://backpack.tf/profiles/76561198050433855#!/compare/1522281600/1522368000

Buyer: https://backpack.tf/profiles/76561198288970487#!/compare/1522368000/1522713600

Gonna be pretty high without a B/O, excluding.

Sale 9:

Bulk for Scorching Flames Pyromancer's Mask

Seller: https://backpack.tf/profiles/76561198035458322#!/compare/1520812800/1520899200

Buyer:https://backpack.tf/profiles/76561198071983249#!/compare/1520812800/1520899200

Sale 10:

Bulk for Morning Glory Valley Forge

Seller: https://backpack.tf/profiles/76561198288970487#!/compare/1522368000/1522713600

Buyer: https://backpack.tf/profiles/76561198074924284#!/compare/1522627200/1522713600

Sale 11:

Bulk for things

Seller: https://backpack.tf/profiles/76561198086948995#!/compare/1524528000/1524700800

Buyer: https://backpack.tf/profiles/76561198058742814#!/compare/1524614400/1524700800

Sellers:

25 keys:

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

https://backpack.tf/item/6672004753

https://i.imgur.com/Vb8H2zP.png

https://backpack.tf/item/4270286766

~25.6 keys:

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

https://backpack.tf/item/4959815649

~25.9 keys:

https://i.imgur.com/oc8L6Dc.png

https://backpack.tf/item/4260703296

32 keys:

https://i.imgur.com/L7FiaRf.png

https://backpack.tf/item/5076340000

35 keys:

https://i.imgur.com/euypph9.png

https://backpack.tf/item/4621941023

40 keys:

https://i.imgur.com/xszp4SH.png

https://backpack.tf/item/4264049295

Sales at:

22

24 (3x)

25

Bot.tf stats also show a bot purchase at ~24 keys: https://i.imgur.com/ZWLNLZk.png

22 is a bit too close to buyers. Excluding 22.

Going with 24-25.