Sale #1:

https://backpack.tf/profiles/76561198104444235#!/compare/1550707200/1550880000

Untraceable.

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Sale #2:

https://backpack.tf/profiles/76561198047941585#!/compare/1550793600/1550880000

Sold for 17 keys.

17 keys

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Sale #3:

https://backpack.tf/profiles/76561198036558300#!/compare/1550880000/1551052800

Untraceable.

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Sale #4:

https://marketplace.tf/items/463;5;u3019

Sold for $55.

$55 / $1.80 = ~31 keys

31 keys

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Sale #5:

https://bot.tf/stats/item?def=463&q=5&ef=Poisonous+Bubbles+of+Regret&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198340634356#!/compare/1550962800/1551049200

Sold for 18 keys.

18 keys

Sales:

17, 18, 31. Going with 17 - 18 keys. 31 looks like an outlier.