Sale #1:

https://backpack.tf/profiles/76561198182342051#!/compare/1543363200/1543449600

Sold for 5 keys and 70 ref (6.75 keys)

~7 keys

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Sale #2:

https://marketplace.tf/items/30836;5;u31

Sold on December 30, 2018 for $15.99

$15.99 / $1.77 = ~9 keys

9 keys

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Sale #3:

https://marketplace.tf/items/30836;5;u31

Sold on December 28, 2018 for $12.

$12 / $1.79 = ~6.5 keys

6.5 keys

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Sale #4:

https://marketplace.tf/items/30836;5;u31

Sold on December 25, 2018 for $15.26

$15.26 / $1.80 = ~8.5 keys

8.5 keys

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Sale #5:

https://marketplace.tf/items/30836;5;u31

Sold on December 17, 2018 for $12.

$12 / $1.65 = ~7 keys

7 keys

Buyers above 7 keys counters a 7-key low end (https://i.imgur.com/fmQtAhn.png). Seller at 8 keys for 2 months (https://i.imgur.com/fxzocuO.png) shows 8 keys should be the low end.