Sale #1:

https://backpack.tf/profiles/76561198021055079#!/compare/1552608000/1552694400

Sold for ~16 keys.

16 keys

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Sale #2:

https://backpack.tf/profiles/76561198008073880#!/compare/1553299200/1553385600

Bulk.

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Sale #3:

https://backpack.tf/profiles/76561198119634686#!/compare/1547510400/1547596800

Untraceable.

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Sale #4:

https://bot.tf/stats/item?def=30621&q=5&ef=Poisonous+Bubbles+of+Regret&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198335791307#!/compare/1547593200/1547679600

Bought for 8 keys.

8 keys

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Sale #5:

https://backpack.tf/profiles/76561198095772247#!/compare/1547942400/1548288000

Sold for 7 keys.

7 keys

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Sale #6:

https://backpack.tf/profiles/76561198124728984#!/compare/1547856000/1547942400

Bulk.

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Sale #7:

https://marketplace.tf/items/30621;5;u3019

Sold for $20.

$20 / $1.80 = ~11 keys

11 keys

Sales:

7, 8, 11, 16. Going with 8 - 11 keys because 7 is quickbuying price.