Sale 1:

Sold for Killstreak Australium Sniper Rifle (~25)

Seller: https://backpack.tf/profiles/76561198317606171#!/compare/1544140800/1544227200

Buyer: https://backpack.tf/profiles/76561198147254211#!/compare/1544140800/1544227200

https://backpack.tf/item/280527366

https://i.imgur.com/hE7bPs7.png - B/O of 25

X = 25

Sale 2:

Sold for Haunted Ghosts Larrikin Robin (18) + ~5 keys in pure/items

Seller: https://backpack.tf/profiles/76561198113853563#!/compare/1549152000/1549238400

Buyer: https://backpack.tf/profiles/76561198079756509#!/compare/1549238400/1551312000

https://backpack.tf/item/6864105207

X = 23

Sale 3:

Sold for 23 keys

Seller: https://backpack.tf/profiles/76561198317606171#!/compare/1548892800/1548979200

Buyer: https://backpack.tf/profiles/76561198117923240#!/compare/1548547200/1548979200

https://backpack.tf/item/6864105207

X = 23

Sale 4:

Sold for 20 keys more than half a key in items

Seller: https://backpack.tf/profiles/76561197991111981#!/compare/1546560000/1547078400

Buyer: https://backpack.tf/profiles/76561198348535426#!/compare/1546992000/1547078400

https://backpack.tf/item/6864105207

X = 21

Sale 5:

Sold with cheap stranges for 21 keys

Seller: https://backpack.tf/profiles/76561197970562873#!/compare/1550880000/1550966400

Buyer: https://backpack.tf/profiles/76561198100912739#!/compare/1550880000/1550966400

https://backpack.tf/item/6864105207

X = 21

Sale 6:

Sold for 21 keys

Seller: https://backpack.tf/profiles/76561198148843171#!/compare/1551225600/1551312000

Buyer: https://backpack.tf/profiles/76561198147596660#!/compare/1551225600/1551312000

https://backpack.tf/item/6864105207

X = 21

Sale 7:

Sold for 19 keys + 22 ref (Buyer’s end)

Seller: https://backpack.tf/profiles/76561198348535426#!/compare/1547078400/1547164800

Buyer: https://backpack.tf/profiles/76561198317606171#!/compare/1547078400/1547164800

https://backpack.tf/item/6864105207

https://i.imgur.com/hE7bPs7.png - bot.tf confirms

X = 20 (at the time)

Sale 8:

Sold for 20 keys

Seller: https://backpack.tf/profiles/76561198087058721#!/compare/1544054400/1544140800

Buyer: https://backpack.tf/profiles/76561198280357510#!/compare/1544054400/1544140800

https://backpack.tf/item/934225286

X = 20

Sale 9:

Sold for 19 keys + 6 ref

Seller: https://backpack.tf/profiles/76561198174361109#!/compare/1543968000/1544140800

Buyer: https://backpack.tf/profiles/76561198317606171#!/compare/1543968000/1544054400

https://backpack.tf/item/280527366

X = 19

Sale 10:

Sold for 19 keys + 8 ref

Seller: https://backpack.tf/profiles/76561198062727900#!/compare/1545004800/1545091200x

Buyer: https://backpack.tf/profiles/76561198080179568#!/compare/1545004800/1545091200

https://backpack.tf/item/6864105207

X = 19

Sale 11:

Sold for 18 keys + 2 ref

Seller: https://backpack.tf/profiles/76561198100912739#!/compare/1551052800/1551139200

Buyer: https://backpack.tf/profiles/76561198148843171#!/compare/1551052800/1551139200

https://backpack.tf/item/6864105207

X = 18

Sale 12:

Sold for Disco Beat Down Familiar Fez + ~2 keys in pure/items

Seller: https://backpack.tf/profiles/76561198079756509#!/compare/1549152000/1549238400

Buyer: https://backpack.tf/profiles/76561198379270361#!/compare/1549238400/1549324800

https://backpack.tf/item/6864105207

Should fit in the range/support the high-end.

Sale 13:

Sold for Kill-a-Watt Tartan Spartan + 9 keys

Seller: https://backpack.tf/profiles/76561198379270361#!/compare/1550880000/1550966400

Buyer: https://backpack.tf/profiles/76561197970562873#!/compare/1550880000/1550966400

https://backpack.tf/item/6864105207

Should fit in the range/support the high end.

Marketplace Sales:

https://marketplace.tf/items/158;5;u9

https://manic.tf/keyprice/?at=2018-12-27 - $27.25 / $1.79 = 15.22 keys

https://manic.tf/keyprice/?at=2019-01-05 - $39.99 / $1.77 = 22.59 keys

X = 23

X = 15

Other Sales:

https://backpack.tf/item/6864105207 (#1-7)

https://backpack.tf/item/934225286 (#8 & #9, the rest are gambling)

https://backpack.tf/item/403825553 (#10)

#1

Untraceable

https://backpack.tf/profiles/76561198317606171#!/compare/1547942400/1548115200

https://backpack.tf/profiles/76561198100622727#!/compare/1547942400/1548115200

https://i.imgur.com/hE7bPs7.png - Labelled as 23 keys, only a couple keys match.

#2

Untraceable

https://backpack.tf/profiles/76561198100622727#!/compare/1548201600/1548374400

https://backpack.tf/profiles/76561198317606171#!/compare/1548115200/1548201600

https://i.imgur.com/hE7bPs7.png - Labelled as 20 keys + 8 ref

#3

Sold for Purple Confetti Winter Woodsman

https://backpack.tf/profiles/76561198117923240#!/compare/1548979200/1549065600

https://backpack.tf/profiles/76561198113853563#!/compare/1549065600/1549152000

Using on the woodsman.

#4

Untraceable

https://backpack.tf/profiles/76561198080179568#!/compare/1545523200/1545609600

https://backpack.tf/profiles/76561198317606171#!/compare/1545696000/1545782400

https://i.imgur.com/hE7bPs7.png - labelled as 19 keys + 6 ref

Probably missing a person in the history

#5

Untraceable

https://backpack.tf/profiles/76561198317606171#!/compare/1545955200/1546041600

https://backpack.tf/profiles/76561198088171813#!/compare/1545955200/1546041600

https://i.imgur.com/hE7bPs7.png - labelled as 23 keys

#6

Untraceable

https://backpack.tf/profiles/76561198251214662#!/compare/1546387200/1546473600

https://backpack.tf/profiles/76561198175420885#!/compare/1546387200/1546473600

#7

Untraceable

https://backpack.tf/profiles/76561198175420885#!/compare/1546387200/1546473600

https://backpack.tf/profiles/76561197991111981#!/compare/1545004800/1546473600

#8

Sold with keys for Secret Snaggletooth

https://backpack.tf/profiles/76561198280357510#!/compare/1544054400/1544140800

https://backpack.tf/profiles/76561198101092551#!/compare/1544054400/1544140800

#9

Untraceable

https://backpack.tf/profiles/76561198101092551#!/compare/1544140800/1544227200

https://backpack.tf/profiles/76561198041731248#!/compare/1544140800/1544227200

#10

Untraceable

https://backpack.tf/profiles/76561198024867201#!/compare/1543881600/1543968000

https://backpack.tf/profiles/76561198072164101#!/compare/1543881600/1543968000

Sellers:

https://i.imgur.com/atz6XcU.png

https://backpack.tf/item/6864105207 - 27 keys

Sales at:

15

18

19 (2x)

20 (2x)

21 (3x)

23 (3x)

25

Sales at or below current buyers are far too common to be excluded. Keeping 20, but excluding those under 20.

Going with 20-25.