Sale #1:

https://marketplace.tf/items/1107;5;u3017

Sold for $22.

$22 / $1.81 = ~12 keys

12 keys

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Sale #2:

https://backpack.tf/profiles/76561198075074962#!/compare/1547596800/1547683200

Sold for 8 keys.

8 keys

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Sale #3:

https://backpack.tf/profiles/76561198352738720#!/compare/1548374400/1548460800

Sold for 7 keys

7 keys

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Sale #4:

https://backpack.tf/profiles/76561198124728984#!/compare/1549065600/1549152000

Sold for an Atomic A Head Full of Hot Air (10.5 keys)

11 keys

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Sale #5:

https://backpack.tf/profiles/76561198861333422#!/compare/1550102400/1550188800

Using in another suggestion.

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Sale #6:

https://backpack.tf/profiles/76561198342476957#!/compare/1546128000/1546214400

Sold for a Circling Peace Sign Puffy Polar Cap (10.5 keys)

11 keys

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Sale #7:

https://backpack.tf/profiles/76561198325571264#!/compare/1546128000/1546214400

Sold for 8 keys.

8 keys

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Sale #8:

https://backpack.tf/profiles/76561198325571264#!/compare/1546128000/1546214400

Unsure.

12 looks like it is too high based on current sellers, 11 keys is too common not to include.