4 sales

1) 8 keys?

https://backpack.tf/profiles/76561198031090822#!/compare/1550102400/1550188800

https://backpack.tf/profiles/76561198045607732#!/compare/1550016000/1550102400

Hmm... Probably involved more than 8 keys, but both sides show only 7 and 8 keys.

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2) Kill-a-Watt Defiant Spartan (18)

https://backpack.tf/profiles/76561198328467289#!/compare/1549670400/1549756800

KaW Spartan sold for 18 keys

https://backpack.tf/profiles/76561198174361109#!/compare/1550707200/1550880000

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3) ?

https://backpack.tf/profiles/76561198328467289#!/compare/1545609600/1545696000

https://backpack.tf/profiles/76561198280176587#!/compare/1545523200/1545868800

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4) $49.99 on January 19th, keys were $1.79

https://marketplace.tf/items/30128;5;u62

49.99/1.79 ≈ 28 keys

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Total: 7, 18, 28

If you will find other sales, you're welcome to report them.

Been a month already.

New sale: $46.82 on April 12th, keys were $1.77

https://marketplace.tf/items/30128;5;u62

46.82/1.77 ≈ 26.5 keys

Close enough to 28.