Sale #1:

https://backpack.tf/profiles/76561198021206951#!/compare/1547769600/1547856000

Sold for 20 keys and 8 ref.

20 keys

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Sale #2:

https://backpack.tf/profiles/76561198227756380#!/compare/1547596800/1547683200

Bulk.

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Sale #3:

https://backpack.tf/profiles/76561198313538268#!/compare/1546556400/1546642800

https://bot.tf/stats/item?def=137&q=5&ef=Cloud+9&craft=1&aus=0&ks=0

Sold for 22 keys.

22 keys

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Sale #4:

https://backpack.tf/profiles/76561198313538268#!/compare/1550962800/1551049200

https://bot.tf/stats/item?def=137&q=5&ef=Cloud+9&craft=1&aus=0&ks=0

Sold for 23 keys.

23 keys

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Sale #4:

https://backpack.tf/profiles/76561198313538268#!/compare/1546124400/1546210800

https://bot.tf/stats/item?def=137&q=5&ef=Cloud+9&craft=1&aus=0&ks=0

Sold for 20 keys.

20 keys

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Sale #5:

https://backpack.tf/profiles/76561198313538268#!/compare/1548025200/1548111600

https://bot.tf/stats/item?def=137&q=5&ef=Cloud+9&craft=1&aus=0&ks=0

Sold for 20 keys.

20 keys

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Sale #6:

https://backpack.tf/profiles/76561198313538268#!/compare/1551135600/1551222000

https://bot.tf/stats/item?def=137&q=5&ef=Cloud+9&craft=1&aus=0&ks=0

Sold for 20 keys.

20 keys

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Sale #7:

https://backpack.tf/profiles/76561198164306336#!/compare/1551312000/1552176000

Unsure.

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Sale #8:

https://backpack.tf/profiles/76561198162277492#!/compare/1552003200/1552089600

Sold for 19 keys?

19 keys

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Sale #9:

https://backpack.tf/profiles/76561198299237697#!/compare/1547596800/1548806400

Untracable.

I'm not too sure about the 19 key sale, and it seems that 20 - 23 is the most common trading point.