Sale #1:

https://backpack.tf/profiles/76561198125791160#!/compare/1548633600/1548720000

Sold for 5 keys.

5 keys

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Sale #2:

https://backpack.tf/profiles/76561198150210219#!/compare/1547164800/1547251200

Bulk.

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Sale #3:

https://backpack.tf/profiles/76561198137743898#!/compare/1553817600/1553990400

Untraceable.

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Sale #4:

https://bot.tf/stats/item?def=30876&q=5&ef=Poisonous+Bubbles+of+Regret&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198314211872#!/compare/1549234800/1549321200

Bought for 3 keys and 16 ref.

3.5 keys

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Sale #5:

https://backpack.tf/profiles/76561198208157121#!/compare/1551312000/1551484800

Untraceable.

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Sale #6:

https://marketplace.tf/items/30876;5;u3019

Sold for $14.02.

$14.02 / $1.80 = ~8 keys

8 keys

Sales:

3.5, 5, 8. Going with 5 keys, seller at 6 keys shows 8 is likely an outlier.