Alright, I think I understand how these work now.

Sale #1:

https://marketplace.tf/items/318;5;u30

https://i.imgur.com/MGfm74s.png

Sold for $16.09

$16.09 / $1.75 = ~9.2 keys

9 keys

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Sale #2:

https://backpack.tf/profiles/76561198075993363#!/compare/1548201600/1548288000

Sold for 9 keys and 18 ref

~9.5 keys

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Sale #3:

https://backpack.tf/profiles/76561198075993363#!/compare/1547078400/1547683200

Sold for 11 keys

11 keys

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Sale #4:

https://backpack.tf/profiles/76561198061275415#!/compare/1545264000/1545350400

Sold with 6 keys and 8 ref for a Green Confetti Tundra Top (16.5 keys)

16.5 - 6.2 = 10.3

~10 keys

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Minis:

Green Confetti Tundra Top:

https://backpack.tf/profiles/76561198133467786#!/compare/1544918400/1545004800

Sold for a Cloud 9 Grenadier's Softcap (16.5 keys)

16.5 keys

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Cloud 9 Grenadier's Softcap:

https://backpack.tf/profiles/76561198141753331#!/compare/1545782400/1545868800

Sold for 16 keys and 23 ref

16.5 keys

Going with 10 - 11 keys as quickbuyers are @ around 9.5 keys.