Using last month.

Sale 1:

Sold for 13 keys + 20 ref

Seller: https://backpack.tf/profiles/76561198244700384#!/compare/1526169600/1526256000

Buyer: https://backpack.tf/profiles/76561198061248329#!/compare/1526169600/1526256000

X = 14

Sale 2:

Sold to a bot for 13 keys

Seller: https://backpack.tf/profiles/76561198090389125#!/compare/1524873600/1524960000

Buyer: https://backpack.tf/profiles/76561198344933095#!/compare/1524873600/1524960000

Bot.tf confirms: https://i.imgur.com/PueDF8J.png

X = 13

Sale 3:

Sold to a quickbuyer for 12 keys

Seller: https://backpack.tf/profiles/76561198057823292#!/compare/1524873600/1525305600

Buyer: https://backpack.tf/profiles/76561198002757046#!/compare/1525219200/1525305600

X = 12

Sale 4:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1525305600/1525392000

Buyer: https://backpack.tf/profiles/76561198253093948#!/compare/1525305600/1525392000

X = 11

Sale 5:

Sold to a quickbuyer for 11 keys

Seller: https://backpack.tf/profiles/76561198058939656#!/compare/1526601600/1526688000

Buyer: https://backpack.tf/profiles/76561198002757046#!/compare/1526601600/1526688000

X = 11

Sale 6:

Bulk for Haunted Ghosts Boxcar Bomber

Seller: https://backpack.tf/profiles/76561198052984141#!/compare/1524873600/1524960000

Buyer: https://backpack.tf/profiles/76561198090389125#!/compare/1524873600/1524960000

Sellers:

12 keys:

https://i.imgur.com/e1960oY.png

https://backpack.tf/item/4446152055

15 keys:

https://i.imgur.com/e1960oY.png

https://backpack.tf/item/4415703160

Sales at:

11 (2x)

12

13

14

Going with 11-14.