Sale 1:

Sold for 18 keys

Seller: https://backpack.tf/profiles/76561198114402400#!/compare/1550793600/1550880000

Buyer: https://backpack.tf/profiles/76561198014468512#!/compare/1550620800/1550880000

https://backpack.tf/item/196074061

X = 18

Sale 2:

Sold for 13 keys

Seller: https://backpack.tf/profiles/76561198027146552#!/compare/1547164800/1547251200

Buyer: https://backpack.tf/profiles/76561198072583900#!/compare/1547164800/1547251200

https://backpack.tf/item/277722296

X = 13

Marketplace Sales:

https://marketplace.tf/items/248;5;u7

https://manic.tf/keyprice/?at=2019-01-02 - $29.99 / $1.77 = 16.94 keys

https://manic.tf/keyprice/?at=2019-02-09 - $33 / $1.82 = 18.13 keys

$18.01 sale would’ve gone to a buy order, excluding.

X = 18

X = 17

Other Sales:

https://backpack.tf/item/277722296 (#1 & #2)

https://backpack.tf/item/196074061 (#3-8)

#1

Untraceable, only some keys match

https://backpack.tf/profiles/76561198819564432#!/compare/1546560000/1546905600

https://backpack.tf/profiles/76561198027146552#!/compare/1546819200/1546992000

#2

Untraceable

https://backpack.tf/profiles/76561198072583900#!/compare/1547251200/1549670400

https://backpack.tf/profiles/76561198131066295#!/compare/1547769600/1548633600

#3

Bulk

https://backpack.tf/profiles/76561198107307547#!/compare/1546732800/1546819200

https://backpack.tf/profiles/76561198232041512#!/compare/1546732800/1546819200

#4

Bulk

https://backpack.tf/profiles/76561198232041512#!/compare/1547164800/1547251200

https://backpack.tf/profiles/76561198013245606#!/compare/1547164800/1547251200

#5

Bulk for Pan

https://backpack.tf/profiles/76561198013245606#!/compare/1547856000/1547942400

https://backpack.tf/profiles/76561198071805624#!/compare/1547856000/1547942400

#6

Untraceable

https://backpack.tf/profiles/76561198071805624#!/compare/1549584000/1549756800

https://backpack.tf/profiles/76561198066742724#!/compare/1549238400/1549843200

Probably sold on marketplace

#7

Untraceable

https://backpack.tf/profiles/76561198073522694#!/compare/1550102400/1550188800

https://backpack.tf/profiles/76561198419584303#!/compare/1550102400/1550188800

#8

Untraceable

https://backpack.tf/profiles/76561198419584303#!/compare/1550102400/1550188800

https://backpack.tf/profiles/76561198114402400#!/compare/1550102400/1550188800

Sellers:

https://i.imgur.com/MBJHs8l.png

https://backpack.tf/item/379785619 - 18 keys

Sales at:

13

17

18 (2x)

13 looks like an outlier, and only took about 3 days to sell, excluding 13.

Going with 17-18.