Only using sales within the last 30 days, as they should be sufficient here.

Sale 1:

Sold for 12 keys + 19 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1515110400/1515196800

Buyer: https://backpack.tf/profiles/76561198147398549#!/compare/1514937600/1515196800

X = 13

Sales 2 & 3:

Sold for $22 and $23.99 on marketplace.tf

https://marketplace.tf/items/30614;5;u3007

Couldn’t find histories

22/1.95 = ~11.3

23.99/1.9 = ~12.6

X = 11 & 13

Sale 4:

Sold for Smoking Brain-Warming Wear (Mini at 11 below) + cheap cosmetics

No outpost

Seller: https://backpack.tf/profiles/76561198247205832#!/compare/1515196800/1515283200

Buyer: https://backpack.tf/profiles/76561198338418472#!/compare/1515196800/1515283200

X = 11

Sale 5: – Credit to Erik

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198091502579#!/compare/1514937600/1515024000

Buyer: https://backpack.tf/profiles/76561198069616176#!/compare/1514592000/1515024000

https://gyazo.com/b287138597f35a933bac952cb0248145

https://i.imgur.com/rgHe3sb.png - Big thanks to Erik for the screenshots

X = 10

Sale 6:

Bulk for Neutron Star Nasty Norsemann

Seller: https://backpack.tf/profiles/76561198323993498#!/compare/1514764800/1514851200

Buyer: https://backpack.tf/profiles/76561198136702159#!/compare/1514764800/1514851200

Smoking Brain-Warming Wear Mini:

Sold for $21 on Marketplace.tf

https://marketplace.tf/items/30827;5;u35

21/1.96 = ~10.7

X = 11

Sellers:

10 keys:

https://i.imgur.com/KOkRbro.png - 6 days

https://i.imgur.com/LreaKOp.png - 5 days

~11.3 keys:

https://i.imgur.com/JgRZjBx.png - 9 days

15 keys:

https://i.imgur.com/HytZcxG.png - 2x, 2 months

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

Literally not a single one on the SCM.

Sales at:

10

11 (2x)

13 (2x)

Going with 10-13.