History 1: (Pro KS) https://backpack.tf/item/6577174176

Untraceable:

https://backpack.tf/profiles/76561198810351708#!/compare/1606089600/1606176000

https://backpack.tf/profiles/76561199013548287#!/compare/1606089600/1606780800

History 2 (spec KS): https://backpack.tf/item/3912602599

https://marketplace.tf/items/tf2/326;14;kt-2

December 21: $19.9/2.01=9.9 keys

History 3: (festivized pro KS) https://backpack.tf/item/3728814659

untraceable:

https://backpack.tf/profiles/76561198011707565#!/compare/1607299200/1607472000

https://backpack.tf/profiles/76561198959811403#!/compare/1607299200/1607731200

untraceable:

https://backpack.tf/profiles/76561198959811403#!/compare/1607990400/1609804800

https://backpack.tf/profiles/76561198350879574#!/compare/1607990400/1608076800

untraceable:

https://backpack.tf/profiles/76561198350879574#!/compare/1609891200/1609977600

https://backpack.tf/profiles/76561198124723591#!/compare/1609891200/1609977600

dumped to Quicksell.store for 6 keys 13 ref

https://backpack.tf/profiles/76561198124723591#!/compare/1609977600/1610150400

https://backpack.tf/profiles/76561199085116273#!/compare/1610064000/1610150400

untraceable:

https://backpack.tf/profiles/76561199085116273#!/compare/1610841600/1610928000

https://backpack.tf/profiles/76561198285887364#!/compare/1610841600/1610928000

Untraceable:

https://backpack.tf/profiles/76561198285887364#!/compare/1610928000/1611014400

https://backpack.tf/profiles/76561199001949552#!/compare/1610841600/1610928000

History 4: (pro ks) https://backpack.tf/item/2669914965

Tradeback, didn't actually sell on marketplace.tf

History 5: https://backpack.tf/item/2400638774

tradebacks

History 6: https://backpack.tf/item/2376529004

tradebacks

History 7: (duped) https://backpack.tf/item/2330885964

Untraceable:

https://backpack.tf/profiles/76561198130182991#!/compare/1598918400/1605225600

https://backpack.tf/profiles/76561198015687452#!/compare/1598400000/1607904000

Untraceable:

https://backpack.tf/profiles/76561198015687452#!/compare/1598400000/1607904000

https://backpack.tf/profiles/76561198035512290#!/compare/1607904000/1607990400

Other sales:

https://marketplace.tf/items/tf2/326;14 (no ks)

November 2: $22.22/1.82=12.21 ~12.2 keys

https://steamcommunity.com/market/listings/440/Collector%27s%20Specialized%20Killstreak%20Back%20Scratcher (spec KS)

November 28 2 AM: $12.05/2.27=5.3 keys

https://steamcommunity.com/market/listings/440/Collector%27s%20Festivized%20Professional%20Killstreak%20Back%20Scratcher (festivized pro KS)

January 3 6 AM: $23/2.34=9.82 ~9.8 keys

Sales at 5.3, ~6.25, 9.8, 9.9, 12.2

5.3 and 6.25 are lower than buyers so excluding

Seller at 12 has been there for 5 days https://backpack.tf/item/5461662434

12.2 was on a non ks and 9.8-9.9 was on ks versions, 12.2 looks like an outlier. Also given the fact we're excluding 2 lower sales, excluding this value too would balance out the range. it's also the oldest sale

Taking 9.8-9.9 keys

Pro KS seller at 12 keys for 5 months that supports excluding 12.2 https://backpack.tf/item/2911982111 (I forgot to include this in the suggestion, my bad)