Sale 1:

Sold for $24.49 on Marketplace.tf

https://marketplace.tf/items/330;5;u11

https://backpack.tf/item/280516261

24.49/1.9 = ~12.9

X = 13

Sale 2:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198150783829#!/compare/1507852800/1507939200

Buyer: https://backpack.tf/profiles/76561198254261867#!/compare/1507852800/1507939200

X = 11

Sale 3:

Sold for Tesla Wing Mann

http://www.tf2outpost.com/trade/29382818 - No visible B/O, nothing cached

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1506643200/1506729600

Buyer: https://backpack.tf/profiles/76561198254469336#!/compare/1506643200/1506729600

B/O was lower without a doubt. Excluding this sale.

Sellers:

10 keys?:

http://www.tf2outpost.com/trade/29447935 - “I got scammed...”

11 keys:

https://i.imgur.com/lwfK67I.png - 2 months

Offers:

http://www.tf2outpost.com/trade/29447083 - 1 week

Sales at:

11

13

13 is an obvious outlier with the seller at 11. Excluding 13.

Going with 11 flat.