Sale :

Quicksold for 5 keys + 14 ref

Seller: https://backpack.tf/profiles/76561198061110181#!/compare/1550966400/1551052800

Buyer: https://backpack.tf/profiles/76561198317903441#!/compare/1550966400/1551052800

https://backpack.tf/item/6324498385

X = 5.5

Marketplace Sales:

https://marketplace.tf/items/1116;5;u3014

https://manic.tf/keyprice/?at=2019-03-08 - $18.4 / $1.8 = 10.22 keys

https://marketplace.tf/items/5021;6/Mann%20Co.%20Supply%20Crate%20Key - 11.6/1.8 = 6.444

X = 10

X = 6.5

Other Sales:

https://backpack.tf/item/6324498385 (#1)

https://backpack.tf/item/2805476566 (#2 & #3)

https://backpack.tf/item/2720924188 (#4)

https://backpack.tf/item/2702934613 (#5 & #6)

https://backpack.tf/item/5139472439 - scammers

https://backpack.tf/item/4607377351 - nothing usable

https://backpack.tf/item/4051769200 - tradeit

#1

Untraceable

https://backpack.tf/profiles/76561198044335614#!/compare/1550880000/1550966400

https://backpack.tf/profiles/76561198061110181#!/compare/1550966400/1551052800

#2

SCM

https://backpack.tf/profiles/76561198205415264#!/compare/1551312000/1551484800

https://backpack.tf/profiles/76561198012537041#!/compare/1551657600/1551830400

#3

Untraceable

https://backpack.tf/profiles/76561198012537041#!/compare/1551830400/1552348800

https://backpack.tf/profiles/76561198124728984#!/compare/1552262400/1552348800

#4

Untraceable

https://backpack.tf/profiles/76561198223191956#!/compare/1551916800/1552003200

https://backpack.tf/profiles/76561198055622215#!/compare/1551916800/1552003200

#5

Untraceable

https://backpack.tf/profiles/76561198077721723#!/compare/1548806400/1549152000

https://backpack.tf/profiles/76561198161748319#!/compare/1548979200/1549411200

#6

Untraceable

https://backpack.tf/profiles/76561198161748319#!/compare/1550793600/1550880000

https://backpack.tf/profiles/76561197995378859#!/compare/1550620800/1550880000

Sellers:

https://i.imgur.com/Bt1IQc9.png

https://backpack.tf/item/6324498385 - ~9.9 keys

https://backpack.tf/item/5270413889 - ~10.4 keys

Sales at:

5.5

6.5

10

5.5 quicksold, and there are buyers for a bit higher. Excluding 5.5.

Going with 6.5-10.