Sale 1:

Sold for 19 keys + buds (~2)

Seller: https://backpack.tf/profiles/76561198054758757#!/compare/1520467200/1520553600

Buyer: https://backpack.tf/profiles/76561198297577901#!/compare/1520553600/1520640000 (no proper compare link available)

X = 21

Sale 2:

Sold for 11 keys + 25 ref

Seller: https://backpack.tf/profiles/76561198311820950#!/compare/1520640000/1520726400

Buyer: https://backpack.tf/profiles/76561197968968337#!/compare/1520640000/1520726400

X = 12

Sale 3:

Sold for 11 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198114545227#!/compare/1521072000/1521158400

Buyer: https://backpack.tf/profiles/76561198066171590#!/compare/1521158400/1521244800

X = 11

Sale 4:

Sold for 10 keys + 18 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1522195200/1522281600

Buyer: https://backpack.tf/profiles/76561198050904212#!/compare/1522281600/1522454400

Bot.tf confirms (also shows that it was bought for 8 keys): https://i.imgur.com/iDslfUV.png

X = 11

Sale 5:

Sold for 8 keys + 2.11 ref

Seller: https://backpack.tf/profiles/76561198100894341#!/compare/1519948800/1520035200

Buyer: https://backpack.tf/profiles/76561198033694310#!/compare/1519948800/1520035200

X = 8

Sale 6:

Quicksold for 8 keys

Seller: https://backpack.tf/profiles/76561197961205411#!/compare/1520208000/1520294400

Buyer: https://backpack.tf/profiles/76561198306553179#!/compare/1520208000/1520294400

X = 8

Sale 7:

Bulk-ish for Hellish Inferno Skullcracher

Seller: https://backpack.tf/profiles/76561197968968337#!/compare/1521158400/1521331200

Buyer: https://backpack.tf/profiles/76561197994697258#!/compare/1521072000/1521158400

Sale 8:

Bulk for Secret VVF

Seller: https://backpack.tf/profiles/76561198022652091#!/compare/1521417600/1521763200

Buyer: https://backpack.tf/profiles/76561198070474543#!/compare/1521676800/1521763200

Sellers:

13 keys:

https://i.imgur.com/M1DnVfp.png

https://backpack.tf/item/2780805693

14 keys:

https://i.imgur.com/CYsyW4I.png

https://backpack.tf/item/2709761675

https://backpack.tf/item/5088421295

30 keys:

http://www.tf2outpost.com/trade/29464244

37 keys:

https://i.imgur.com/lW97xz8.png

https://backpack.tf/item/3522618949

47 keys:

https://i.imgur.com/ZbpeU17.png

https://backpack.tf/item/3493530505

Offers:

http://www.tf2outpost.com/trade/29391670

http://www.tf2outpost.com/trade/29603799

Sales at:

8 (3x)

11 (2x)

12

21

8 went to quickbuyers and/or quicksold, excluding 8.

21 is a large outlier, excluding 21.

Going with 11-12.