The Sale:

Sold to a quickbuyer for 8 keys + 9 ref

Seller: https://backpack.tf/profiles/76561198089231891#!/compare/1551744000/1551830400

Buyer: https://backpack.tf/profiles/76561198006544512#!/compare/1551744000/1551830400

https://backpack.tf/item/270654665

X = 8

Marketplace Sales:

https://marketplace.tf/items/255;5;u7

https://manic.tf/keyprice/?at=2019-01-24 - $15.19 / $1.75 = 8.68 keys

https://manic.tf/keyprice/?at=2019-03-15 - $17.78 / $1.8 = 9.88 keys

X = 10

X = 8.5

Other Sales:

https://backpack.tf/item/917582754 (#1 & #2, the rest were gambled)

https://backpack.tf/item/270654665 (#3)

#1

Sold for 8 keys + 10 ref?

https://backpack.tf/profiles/76561198337734785#!/compare/1547424000/1547510400

https://backpack.tf/profiles/76561198200597703#!/compare/1546992000/1547510400

More would have been involved, but would have been spent on the same day.

#2

Gambled

https://backpack.tf/profiles/76561198042951716#!/compare/1552348800/1552694400

https://backpack.tf/profiles/76561198006544512#!/compare/1552608000/1552694400

#3

Untraceable

https://backpack.tf/profiles/76561198337734785#!/compare/1550966400/1551052800

https://backpack.tf/profiles/76561198056252537#!/compare/1550966400/1551052800

Sellers:

https://i.imgur.com/f5B5S0h.png

https://backpack.tf/item/917582754 - 10 keys

https://backpack.tf/item/270654665 - 10 keys

https://backpack.tf/item/244268447 - 11 keys

https://backpack.tf/item/1840204867 - 11 keys

https://backpack.tf/item/923625473 - ~11.5 keys

Sales at:

8

8.5

10

Quickbuyers at 8+, excluding 8.

Going with 8.5-10.