4 sales

1) 18 keys

https://backpack.tf/profiles/76561198054275169#!/compare/1551398400/1551484800

https://backpack.tf/profiles/76561198323565601#!/compare/1551398400/1551484800

2) ?

https://backpack.tf/profiles/76561198106210616#!/compare/1550707200/1551312000

3) 16 keys?

https://backpack.tf/profiles/76561198323565601#!/compare/1550620800/1550707200

4) $69.99 on 14th February, keys were $1.80

https://marketplace.tf/items/319;5;u29

https://marketplace.tf/items/5021;6/Mann%20Co.%20Supply%20Crate%20Key

69.99/1.8 = ~39 keys

Checked all previous suggestions to find more sales, because I think 4 sales for ~42 existing hats aren't enough, but many hats haven't traded for over a year at least.

https://backpack.tf/item/1145291737

https://backpack.tf/item/1596811528

Total: 16, 18, 39

Excluding 16 and 39 and going only with 18. If you will find other sales - you are welcome to report them.