Sales 1 & 2:

Sold for $13.99 and $11.50 on Marketplace.tf

https://marketplace.tf/items/1117;5;u3007

13.99/1.96 = ~7.14

11.50/1.98 = ~5.81

X = 7

X = 6

Sale 3:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198415535845#!/compare/1525392000/1525478400

Buyer: https://backpack.tf/profiles/76561198070917778#!/compare/1525392000/1525478400

X = 5

Sale 4:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1523664000/1523750400

Buyer: https://backpack.tf/profiles/76561198092449612#!/compare/1523664000/1523750400

X = 5

Sale 5:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198314160142#!/compare/1524009600/1524096000

Buyer: https://backpack.tf/profiles/76561198010245943#!/compare/1524009600/1524096000

X = 5

Sale 6:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198306211537#!/compare/1524787200/1524873600

Buyer can’t be identified - https://backpack.tf/item/5897497634

Untraceable, but bot.tf confirms: https://i.imgur.com/oySLnqh.png

X = 5

Sellers:

~5.6 keys:

https://i.imgur.com/4bZtbOl.png

https://backpack.tf/item/3008502122

https://backpack.tf/item/5035684791

~6 keys:

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

https://backpack.tf/item/3565571532

https://backpack.tf/item/6507307497

9 keys:

https://i.imgur.com/B72BuF2.png

https://backpack.tf/item/2905133991

Offers:

http://www.tf2outpost.com/trade/29583528

http://www.tf2outpost.com/trade/29640148

Sales at:

5 (4x)

6

7

Gonna exclude 7 given the cheaper sellers.

Going with 5-6.