Sale 1:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198011321852#!/compare/1523491200/1523577600

Buyer: https://backpack.tf/profiles/76561198063070081#!/compare/1523577600/1523836800

X = 12

Sales 2, 3, 4 & 5:

Sold for $13.89, $13.78, $13.69 and $13.65 on Marketplace.tf

https://marketplace.tf/items/30673;5;u3008

13.89/1.97 = ~7.05 keys

13.78/1.98 = ~6.96 keys

13.69/1.98 = ~6.91 keys

13.65/1.98 = ~6.89 keys

X = 7 (4x)

Sale 6:

Sold for a lot of items

Seller: https://backpack.tf/profiles/76561198308193624#!/compare/1522972800/1523059200

Buyer: https://backpack.tf/profiles/76561198119246388#!/compare/1522972800/1523059200

Best to try and not choose an exact value from this sale.

Sellers:

~6.95 keys:

https://i.imgur.com/icBq3DC.png

https://backpack.tf/item/6567439359

7 keys:

https://i.imgur.com/RjKYSb1.png

https://backpack.tf/item/4330735290

12 keys:

https://i.imgur.com/P0ZD1yd.png

https://backpack.tf/item/4449635861

Sales at:

7 (4x)

12

In an interesting turn of events, the pure sale ended up being the outlier, rather than the marketplace.tf sales. Excluding 12.

Going with 7 flat.