Sale 1:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198310832681#!/compare/1520726400/1520812800

Buyer: https://backpack.tf/profiles/76561198045199590#!/compare/1520640000/1520812800

X = 9

Sale 2:

Sold for Strange Pro. KS Festive Ambassador (6)

No outpost

Seller: https://backpack.tf/profiles/76561198204198767#!/compare/1522281600/1522368000

Buyer: https://backpack.tf/profiles/76561198093367267#!/compare/1521936000/1522368000

X = 6

Sale 3:

Sold for ~6 keys in items

No outpost

Seller: https://backpack.tf/profiles/76561198355520251#!/compare/1522540800/1522627200

Buyer: https://backpack.tf/profiles/76561198198716181#!/compare/1511654400/1522627200

X = 6

Sale 4:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198067306890#!/compare/1520121600/1520208000

Buyer: https://backpack.tf/profiles/76561198173378059#!/compare/1519862400/1520208000

X = 5

Sale 5:

Sold for 5 keys (~3 day hold)

http://www.tf2outpost.com/trade/29586447 - B/O of 5

Seller: https://backpack.tf/profiles/76561198045199590#!/compare/1520812800/1521072000

Buyer: https://backpack.tf/profiles/76561198254039408#!/compare/1520985600/1521072000

X = 5

Sale 6:

Sold for $10 on Marketplace.tf

https://marketplace.tf/items/30614;5;u3009

Can’t find item history

10/1.98 = ~5.1

X = 5

Sale 7:

Sold for ~4 keys worth of items

Seller: https://backpack.tf/profiles/76561197997283715#!/compare/1521849600/1521936000

Buyer: https://backpack.tf/profiles/76561198204198767#!/compare/1521849600/1521936000

X = 4

Sale 8:

Bulk for Infernal Smoke Rock, Paper, Scissors

Seller: https://backpack.tf/profiles/76561198129966158#!/compare/1520467200/1520553600

Buyer: https://backpack.tf/profiles/76561198204122360#!/compare/1520467200/1520553600

Sale 9:

Bulk for Dead Presidents Hong Kong Cone

Seller: https://backpack.tf/profiles/76561198204122360#!/compare/1520553600/1520640000

Buyer: https://backpack.tf/profiles/76561198111618059#!/compare/1520553600/1520640000

Sellers:

5 keys:

https://i.imgur.com/SQ0G6vV.png

https://backpack.tf/item/4438588450

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

https://backpack.tf/item/3861434154

16 keys:

https://i.imgur.com/vvhKAc5.png

https://backpack.tf/item/3888257694

Sales at:

4

5 (3x)

6 (2x)

9

4 is countered by buyers, excluding.

9 is an outlier, excluding.

Going with 5-6.