Sales 1 & 2:

Sold for $31.99 and $25 on marketplace.tf

https://marketplace.tf/items/1113;5;u3003

https://manic.tf/keyprice/?at=2018-09-16 - $25 / $1.85 = 13.51 keys

https://manic.tf/keyprice/?at=2018-09-28 - $31.99 / $1.83 = 17.48 keys

X = 17

X = 14

Sale 3:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198097271466#!/compare/1539129600/1539216000

Buyer: https://backpack.tf/profiles/76561198309100349#!/compare/1539129600/1539216000

https://backpack.tf/item/3073676877

X = 12

Sale 4:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198335791307#!/compare/1538870400/1538956800

Buyer: https://backpack.tf/profiles/76561198134939963#!/compare/1538870400/1538956800

https://backpack.tf/item/3176635682

https://i.imgur.com/5rIUtFW.png

X = 10

Sale 5:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1538784000/1538870400

Buyer: https://backpack.tf/profiles/76561198154624508#!/compare/1538870400/1538956800

https://backpack.tf/item/6689601077

https://i.imgur.com/5rIUtFW.png

X = 9

Sale 6:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1538352000/1538438400

Buyer: https://backpack.tf/profiles/76561198097271466#!/compare/1538352000/1538438400

https://backpack.tf/item/3073676877

X = 9

Sale 7:

Sold for 8 keys to a bot

Seller: https://backpack.tf/profiles/76561198134939963#!/compare/1539561600/1539648000

Buyer: https://backpack.tf/profiles/76561198335791307#!/compare/1539475200/1539561600

https://backpack.tf/item/3176635682

https://i.imgur.com/5rIUtFW.png

X = 8

Other Sales:

https://backpack.tf/item/6723746561 (#1)

https://backpack.tf/item/6489799626 (#2)

https://backpack.tf/item/3764333730 (#3-5)

https://backpack.tf/item/3073676877 (#6

https://backpack.tf/item/5504020127 - scammer

#1

Sold with 3 keys + some ref for Smoking Trophy Belt

https://backpack.tf/profiles/76561198082041730#!/compare/1538524800/1538611200

https://backpack.tf/profiles/76561198137344104#!/compare/1537747200/1538611200

Better to use on the belt.

#2

Sold, with possible items, for Steaming Noir

https://backpack.tf/profiles/76561198209648320#!/compare/1539129600/1539388800

https://backpack.tf/profiles/76561198798016614#!/compare/1539302400/1539388800

Better to use on the Noir

#3

Bulk for DBD Bloke’s Bucket Hat and a nsfw object

https://backpack.tf/profiles/76561198199959835#!/compare/1538870400/1539043200

https://backpack.tf/profiles/76561198325595749#!/compare/1538870400/1538956800

#4

Bulk for pure

https://backpack.tf/profiles/76561198325595749#!/compare/1538956800/1539043200

https://backpack.tf/profiles/76561198047168424#!/compare/1538870400/1539043200

#5

Untraceable

https://backpack.tf/profiles/76561198047168424#!/compare/1539043200/1539129600

https://backpack.tf/profiles/76561198169132553#!/compare/1539043200/1539129600

#6

Untraceable

https://backpack.tf/profiles/76561197963396109#!/compare/1538352000/1538438400

https://backpack.tf/profiles/76561198002757046#!/compare/1538352000/1538438400

Sellers:

https://i.imgur.com/JJVM6ff.png

https://i.imgur.com/VAj8rDz.png

https://backpack.tf/item/3176635682 - 10 keys

https://backpack.tf/item/6489799626 - 10 keys

https://backpack.tf/item/3073676877 - 10 keys

https://backpack.tf/item/5522156590 - 17 keys

https://backpack.tf/item/2710681164 - 19 keys

https://backpack.tf/item/4079747842 - 19 keys

https://backpack.tf/item/6873992124 - 28 keys

Sales at:

8

9 (2x)

10

12

14

17

8 is a single sale to a quickbuyer, excluding 8.

3 sellers at 10, excluding anything higher than 10.

Going with 9-10.