Using last month.

Sale 1:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198026736504#!/compare/1528156800/1528243200

Buyer: https://backpack.tf/profiles/76561198069944394#!/compare/1528156800/1528243200

X = 12

Sale 2:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198155320889#!/compare/1526774400/1527033600

Buyer: https://backpack.tf/profiles/76561198026736504#!/compare/1527033600/1527120000

X = 6

Sale 3:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198386441509#!/compare/1527206400/1527292800

Buyer is untraceable: https://backpack.tf/item/4459494472

Bot.tf confirms: https://i.imgur.com/KpRoh6E.png

X = 6

Sale 4:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198334598690#!/compare/1528070400/1528156800

Buyer: https://backpack.tf/profiles/76561198199944353#!/compare/1527897600/1528156800

Bot.tf confirms: https://i.imgur.com/KpRoh6E.png

X = 5

Sale 5:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198277345119#!/compare/1526169600/1526256000

Buyer: https://backpack.tf/profiles/76561197979017401#!/compare/1526169600/1526256000

X = 5

Sale 6:

Sold for 4 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198048851745#!/compare/1526688000/1526774400

Buyer: https://backpack.tf/profiles/76561198295506584#!/compare/1526083200/1526774400

X = 4.5

Sale 7:

Sold for 4 keys + 1 ref to a quickbuyer

Seller: https://backpack.tf/profiles/76561198054825186#!/compare/1526688000/1526774400

Buyer: https://backpack.tf/profiles/76561198048851745#!/compare/1526601600/1526688000

X = 4

Sale 8:

Sold for 4 keys + 3 ref

Seller: https://backpack.tf/profiles/76561198026593822#!/compare/1526169600/1526256000

Buyer: https://backpack.tf/profiles/76561198277345119#!/compare/1526169600/1526256000

X = 4

Sale 9:

Sold for Aces High Tin Topi

No outpost

Seller: https://backpack.tf/profiles/76561197979017401#!/compare/1526256000/1526428800

Buyer: https://backpack.tf/profiles/76561198344316001#!/compare/1526342400/1526428800

Gonna be an outlier without a B/O, excluding.

Sale 10:

Bulk-ish for Stormy Helmet Without a Home

Seller: https://backpack.tf/profiles/76561198069944394#!/compare/1528243200/1528329600

Buyer: https://backpack.tf/profiles/76561198168964685#!/compare/1528243200/1528329600

The other bot.tf sale at 5 leads to this: https://backpack.tf/item/4793784070

I’m not too convinced it actually sold for 5 keys, given the bot’s compare link:

https://backpack.tf/profiles/76561198341171690#!/compare/1528070400/1528156800

Though it could’ve just been that the bot owner had withdrawn their items after selling it on the same day. Either way, fits in the range.

Sellers:

[u]Sales at:

4 (2x)

4.5

5 (2x)

6 (2x)

12

12 is an outlier, excluding 12.

Going with 4-6.