Using sales from December onwards.

Sale 1:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198830963112#!/compare/1545523200/1545609600

Buyer: https://backpack.tf/profiles/76561198376090993#!/compare/1545523200/1545609600

https://backpack.tf/item/3360280777

https://i.imgur.com/HgYHZdV.png - bot.tf confirms.

X = 12

Sale 2:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198830963112#!/compare/1544832000/1544918400

Buyer: https://backpack.tf/profiles/76561198120579663#!/compare/1544745600/1544918400

https://backpack.tf/item/6992063846

https://i.imgur.com/HgYHZdV.png - bot.tf confirms.

X = 11

Sale 3:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198830963112#!/compare/1544313600/1544486400

Buyer: https://backpack.tf/profiles/76561198075490415#!/compare/1541376000/1544486400

https://backpack.tf/item/6738487901

https://i.imgur.com/HgYHZdV.png - bot.tf confirms.

X = 10

Sale 4:

Sold for Australium Frontier Justice (9) + 1 key

Seller: https://backpack.tf/profiles/76561198036170813#!/compare/1545264000/1545350400

Buyer: https://backpack.tf/profiles/76561198326910218#!/compare/1545264000/1545350400

https://backpack.tf/item/6125365962

X = 10

Sale 5:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198174084635#!/compare/1544832000/1544918400

Buyer: https://backpack.tf/profiles/76561198148148506#!/compare/1541548800/1544832000

https://backpack.tf/item/6992063846

X = 9

Sale 6:

Sold to a bot for 7 keys + 3 ref?

Seller: https://backpack.tf/profiles/76561198287971918#!/compare/1540598400/1546473600

Buyer: https://backpack.tf/profiles/76561198830963112#!/compare/1544227200/1544313600

https://backpack.tf/item/6738487901

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

We can see exactly 7 keys + 3 ref on the buyer’s end and nothing more. Given the seller’s backpack being unscanned for a while, that would explain why they don’t match, or appear on the seller’s end.

X = 7

Sale 7:

Sold to a bot for 7 keys + 3 ref?

Seller: https://backpack.tf/profiles/76561198148148506#!/compare/1544832000/1546473600

Buyer: https://backpack.tf/profiles/76561198830963112#!/compare/1544745600/1544832000

https://backpack.tf/item/6992063846

https://i.imgur.com/HgYHZdV.png - Labelled as 7 keys + 3 ref

All the ref is visible on the seller’s end, so I doubt anything else would’ve been involved.

X = 7

Sale 8:

Sold to a bot for 7 keys + 3 ref?

Seller: https://backpack.tf/profiles/76561198039731024#!/compare/1545091200/1545177600

Buyer: https://backpack.tf/profiles/76561198830963112#!/compare/1545091200/1545177600

https://backpack.tf/item/3360280777

https://i.imgur.com/HgYHZdV.png - labelled as 7 keys + 3 ref

Similar situation to the sale above, ref matches, doubt bot.tf is wrong.

X = 7

Sale 9:

Sold to a bot for 7 keys + 3 ref

Seller: https://backpack.tf/profiles/76561198288882334#!/compare/1544400000/1544572800

Buyer: https://backpack.tf/profiles/76561198830963112#!/compare/1544486400/1544572800

https://backpack.tf/item/2942382439

https://i.imgur.com/HgYHZdV.png - bot.tf confirms.

X = 7

Other Sales:

https://backpack.tf/item/4467107986 (#1)

https://backpack.tf/item/2942382439 (#2)

https://backpack.tf/item/6493152825 (#3)

https://backpack.tf/item/6487958489 - gambled/mannco

https://backpack.tf/item/4007455736 - not unusual at the time

https://backpack.tf/item/3829327644 - tradeit

#1

Backpack transfer or something of the sort

https://backpack.tf/profiles/76561198818515870#!/compare/1544745600/1545177600

https://backpack.tf/profiles/76561198151005351#!/compare/1541030400/1545177600

#2

Untraceable

https://backpack.tf/profiles/76561198830963112#!/compare/1544745600/1544832000

https://backpack.tf/profiles/76561198173780037#!/compare/1528070400/1544832000

https://i.imgur.com/HgYHZdV.png - labelled as 11 keys, only a couple keys match.

#3

Untraceable

https://backpack.tf/profiles/76561198239513168#!/compare/1543536000/1543622400

https://backpack.tf/profiles/76561198060147112#!/compare/1543536000/1543622400

Sellers:

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

https://backpack.tf/item/3495790266 - 12 keys

https://backpack.tf/item/6992063846 - 14 keys

And another on scrap for ~17 keys.

Sales at:

7 (4x)

9 (1x)

10 (2x)

11

12

7 is way too common to be excluded.

Going with 7-12.