Using last month

Sale 1:

Sold with Hot Case for Aces High Hottie’s Hoodie (13)

Seller: https://backpack.tf/profiles/76561198149773493#!/compare/1523923200/1524009600

Buyer: https://backpack.tf/profiles/76561198419735021#!/compare/1523923200/1524009600

X = 13

Sale 2:

Sold for 7 keys + 32 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1522368000/1522454400

Buyer: https://backpack.tf/profiles/76561198073830880#!/compare/1522368000/1522454400

Bot.t confirms: https://i.imgur.com/fgnLoK4.png

X = 8

Sale 3:

Sold for 7 keys + 26 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1524441600/1524528000

Buyer: https://backpack.tf/profiles/76561198093692174#!/compare/1524441600/1524528000

Untraceable, but bot.tf confirms: https://i.imgur.com/fgnLoK4.png and the dates match up here too.

X = 8

Sale 4:

Untraceable bot.tf sale at 6 keys + 5 ref

https://i.imgur.com/dAZ6M16.png

Clicking the date takes me to the same bot from the other two sales at 8 keys here:

https://backpack.tf/profiles/76561198305676157#!/compare/1524700800/1524787200

Must’ve sold on the day that it was bought.

X = 6

Sale 5:

Bulk for Lantern Chieftain’s Challenge

Seller: https://backpack.tf/profiles/76561197997283715#!/compare/1524182400/1524268800

Buyer: https://backpack.tf/profiles/76561198062749575#!/compare/1524182400/1524268800

Sellers:

15 keys:

https://i.imgur.com/0ItTYax.png

https://backpack.tf/item/6417104597

https://i.imgur.com/mm9mUeM.png

https://backpack.tf/item/5388760799

Sales at:

6

8 (2x)

13

Excluding 6, given that the sale at 6 seems to have sold on the day it was bought, and the SCM buyers at ~6 keys worth - https://i.imgur.com/EamO6vi.png

13 looks like an outlier, excluding 13.

Going with 8 flat.