Using last month

Sale 1:

Sold for 14 keys

Seller: https://backpack.tf/profiles/76561198097271466#!/compare/1523404800/1523491200

Buyer: https://backpack.tf/profiles/76561198128130998#!/compare/1523404800/1523491200

X = 14

Sale 2 & 3:

Sold for $19.99 and $21.99 on Marketplace.tf

https://marketplace.tf/items/30609;5;u3010

19.99/1.98 = ~10.1

21.99/1.98 = ~11.1

X = 11

X = 10

Sale 4:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198094405218#!/compare/1523318400/1523404800

Buyer: https://backpack.tf/profiles/76561198327703767#!/compare/1523318400/1523404800

X = 8

Sale 5:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198097271466#!/compare/1525392000/1525478400

Buyer: https://backpack.tf/profiles/76561198063041674#!/compare/1525392000/1525478400

X = 7

Sale 6:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198086076383#!/compare/1522972800/1523059200

Buyer: https://backpack.tf/profiles/76561198202715615#!/compare/1522972800/1523059200

Buyer’s end shows 6 keys, the 7th key they got from selling their strange gibus to “Items! :D” bot. This key is one of the three that can be seen in the seller’s end.

X = 7

Sale 7:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198097271466#!/compare/1524700800/1524787200

Buyer: https://backpack.tf/profiles/76561198292242424#!/compare/1524700800/1524787200

X = 7

Sale 8:

Bulk for Logo Modest + other items

Seller: https://backpack.tf/profiles/76561198126985704#!/compare/1524355200/1524528000

Buyer: https://backpack.tf/profiles/76561198035620692#!/compare/1524355200/1524528000

Sellers:

30 keys:

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

https://backpack.tf/item/4446692545

Offers:

http://www.tf2outpost.com/trade/29594116

Sales at:

7 (3x)

8

10

11

14

Majority of the sales are at 7 and 8. The sale at 14 is ~25 days old, and is a rather large outlier. Excluding 14.

Sales at 10 and 11 are marketplace.tf sales, and we have plenty of pure sales to work with here already. Excluding 10 and 11.

Going with 7-8.