Using last month

Sale 1:

Sold for 6 keys (Buyer's compare link)

Seller: https://backpack.tf/profiles/76561198089253101#!/compare/1521417600/1521504000

Buyer: https://backpack.tf/profiles/76561198157089186#!/compare/1521504000/1521590400

X = 6

Sale 2:

Sold for 5 keys + ~1 key in metal/items

Seller: https://backpack.tf/profiles/76561198144194194#!/compare/1521504000/1521676800

Buyer: https://backpack.tf/profiles/76561198139267664#!/compare/1521504000/1521676800

X = 6

Sale 3:

Sold for Showstopper Yeti Punch

Seller: https://backpack.tf/profiles/76561198210372209#!/compare/1523577600/1523664000

Buyer: https://backpack.tf/profiles/76561198173381547#!/compare/1523577600/1523664000

Probably better to not use this sale on either taunt, seems like a direct downgrade.

Sale 4:

Bulk for Blizzardy Storm Pyro's Boron Beanie

Seller: https://backpack.tf/profiles/76561198390122058#!/compare/1523404800/1523577600

Buyer: https://backpack.tf/profiles/76561198067055238#!/compare/1523491200/1523577600

Sellers:

~6.5 keys:

https://i.imgur.com/Mt5PJRz.png

https://backpack.tf/item/6252774795

7 keys:

https://i.imgur.com/AcHhRb7.png

https://backpack.tf/item/6253264822

8 keys:

https://i.imgur.com/t4eWWsh.png

https://backpack.tf/item/6252959868

12 keys:

https://i.imgur.com/K1riyfG.png

https://backpack.tf/item/6252728662

22 keys:

https://i.imgur.com/4Y8X22m.png

https://backpack.tf/item/6254692004

Sales at:

6 (2x)

In case these sales weren't enough, here's another sale at ~6 keys:

https://marketplace.tf/items/1182;5;u3008 - more than a month old, but supports the price.

Going with 6 flat.