Sale #1:

https://bot.tf/stats/item?def=1108&q=5&ef=Poisonous+Bubbles+of+Regret&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198335791307#!/compare/1552863600/1552950000

Bought for 5 keys and 25 ref.

5.5 keys

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Sale #2:

https://bot.tf/stats/item?def=1108&q=5&ef=Poisonous+Bubbles+of+Regret&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198088330242#!/compare/1551571200/1551657600

Sold for 7 keys.

7 keys

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Sale #3:

https://backpack.tf/profiles/76561198088330242#!/compare/1552176000/1552262400

Bulk.

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Sale #4:

https://marketplace.tf/items/1108;5;u3019

Sold for $11.38.

$11.38 / $1.80 = ~6.5 keys

6.5 keys

Sales:

5.5, 6.5, 7. Going with 5.5 - 7 keys.