Sale #1:

https://backpack.tf/profiles/76561198055278306#!/compare/1552953600/1553299200

Sold for 23 keys.

23 keys

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Sale #2:

https://marketplace.tf/items/338;5;u16

Sold for $64.44.

$64.44 / $1.75 = ~37 keys

37 keys

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Sale #3:

https://bot.tf/stats/item?def=338&q=5&ef=Vivid+Plasma&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198305494036#!/compare/1550271600/1550358000

Bought for 21 keys and 15 ref.

21 keys

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Sale #4:

https://bot.tf/stats/item?def=338&q=5&ef=Vivid+Plasma&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198305494036#!/compare/1553122800/1553209200

Sold for 26 keys and 15 ref.

26 keys

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Sale #5:

https://bot.tf/stats/item?def=338&q=5&ef=Vivid+Plasma&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198305494036#!/compare/1553209200/1553295600

Bought for 21 keys and 15 ref.

21 keys

Sales:

21, 21, 23, 26, 37. Sales at 21 keys went to quickbuyers, 37 is obviously too high. Going with 23 - 26 keys.