Sale #1:

https://marketplace.tf/items/30753;5;u117

Sold for $54.99.

$54.99 / $1.80 = ~30.6 keys

31 keys

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Sale #2:

https://backpack.tf/profiles/76561198156255538#!/compare/1547078400/1547164800

Sold for 35 keys.

35 keys

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Sale #3:

https://backpack.tf/profiles/76561198010059325#!/compare/1546041600/1546128000

Cash sale?

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Sale #4:

https://backpack.tf/profiles/76561198023476204#!/compare/1544745600/1544832000

Untracable.

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Sale #5:

https://marketplace.tf/items/30753;5;u117

Sold for $155.

$155 / $1.79 = ~87 keys

87 keys

Sales:

31, 35, 87. Seller at 30 keys determines low-end (https://i.imgur.com/J6VLv63.png), going with 30 - 35 keys.