Mini for another suggestion :P

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Sale #1 - Untrace

https://backpack.tf/profiles/76561198337734785#!/compare/1560211200/1560297600

https://backpack.tf/profiles/76561198103937885#!/compare/1560643200/1564185600

Sale #2 - Untrace

https://backpack.tf/profiles/76561198125035818#!/compare/1560211200/1560297600

https://backpack.tf/profiles/76561198837007652#!/compare/1560211200/1560297600

Sale #3 - Untrace

https://backpack.tf/profiles/76561198312583107#!/compare/1560211200/1560297600

https://backpack.tf/profiles/76561198125035818#!/compare/1560211200/1560297600

Sale #4 - https://marketplace.tf/items/tf2/358;5;u34 | https://backpack.tf/item/677098612 | https://bot.tf/stats/listings/Y9GZE0W6

14 / 1.8 = 7.777 keys

capped at 7.68 keys

rounded to 7.5 keys

Sale #5 - Untrace

https://backpack.tf/profiles/76561198419584303#!/compare/1564444800/1564531200

https://backpack.tf/profiles/76561198100248168#!/compare/1564444800/1564531200

Sale #6 - Untrace

https://backpack.tf/profiles/76561198419584303#!/compare/1564444800/1564531200

https://backpack.tf/profiles/76561198287198615#!/compare/1564444800/1564531200

Sale #7 - https://marketplace.tf/items/tf2/358;5;u34 | https://backpack.tf/item/1361040909 | No idea on the b/o

15.5 / 1.84 = 8.4 keys

rounded to 8.5

Sale #8 - untrace

https://backpack.tf/profiles/76561198344023079#!/compare/1563667200/1563840000

https://backpack.tf/profiles/76561198203413492#!/compare/1564272000/1564358400

Sale #9 - Untrace

https://backpack.tf/profiles/76561198203413492#!/compare/1564358400/1565395200

https://backpack.tf/profiles/76561198831479492#!/compare/1563408000/1564358400

Sale #10 - Untrace

https://backpack.tf/profiles/76561198043662502#!/compare/1564185600/1564358400

https://backpack.tf/profiles/76561198831479492#!/compare/1563408000/1564358400

Sale #11 - Untrace

https://backpack.tf/profiles/76561198043662502#!/compare/1564358400/1564444800

https://backpack.tf/profiles/76561198061868800#!/compare/1564272000/1564444800

Sale #12 - Untrace

https://backpack.tf/profiles/76561198323565601#!/compare/1564358400/1564444800

https://backpack.tf/profiles/76561198061868800#!/compare/1564444800/1564531200

Sale #13 - https://marketplace.tf/items/tf2/358;5;u34 | No idea on the id | No idea on the b/o

14 / 1.8 = 7.77, rounded to 8

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Taking 7.5 - 8 keys

Excluding 8.5 due to a seller @ 8 for 6 months - https://gyazo.com/407c65707da85ea9bf3d7d8b1600d9e2