Re-suggesting, missed sales.

Sale 1:

Sold for $26 on Marketplace.tf

https://marketplace.tf/items/323;5;u35

https://backpack.tf/item/494563119

26/2.03 = ~12.8

X = 13

Sale 2:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198106182243#!/compare/1495843200/1495929600

Buyer: https://backpack.tf/profiles/76561198134925169#!/compare/1495843200/1495929600

X = 11

Sale 3:

Sold for 10 keys (IDs match)

No outpost

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1499904000/1499990400

Buyer: https://backpack.tf/profiles/76561198072143188#!/compare/1498262400/1500076800

X = 10

Sale 4:

Sold for 8 keys + 16 ref in ~2-3 days

No outpost

Seller: https://backpack.tf/profiles/76561198217943575#!/compare/1501200000/1501286400

Buyer: https://backpack.tf/profiles/76561198353493333#!/compare/1501200000/1501286400

X = 9

Sale 5:

Quicksold for 9 keys

Seller: https://backpack.tf/profiles/76561198135869440#!/compare/1495929600/1496016000

Buyer: https://backpack.tf/profiles/76561198351844678#!/compare/1495065600/1496016000

X = 9

Sale 6:

Quicksold for 8 keys + 25 ref

No outpost

Seller: https://backpack.tf/profiles/76561198258692045#!/compare/1501372800/1501459200

Buyer: https://backpack.tf/profiles/76561198088878793#!/compare/1501372800/1501459200

X = 9

Sale 7:

Sold for 9 keys + <0.5 keys in metal (Buyer's compare link)

No outpost

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1497744000/1497830400

Buyer: https://backpack.tf/profiles/76561198380918720#!/compare/1497744000/1497830400

X = 9

Sale 8:

Sold with Burly Beast (<3) for Strange Pro. KS AWPer Hand (~11.5)

No outpost

Seller: https://backpack.tf/profiles/76561198102511545#!/compare/1498694400/1498780800

Buyer: https://backpack.tf/profiles/76561198086630630#!/compare/1498694400/1498780800

X = 9

Sale 9:

Sold for Strange Minimal Wear Coffin Nail Grenade Launcher (6) + 2 keys

No outpost

Seller: https://backpack.tf/profiles/76561198353493333#!/compare/1501286400/1501372800

Buyer: https://backpack.tf/profiles/76561198258692045#!/compare/1501286400/1501372800

X = 8

Sale 10:

Bulk for Haunted Ghosts Otolaryngologist's Mirror

Seller: https://backpack.tf/profiles/76561198380918720#!/compare/1497830400/1497916800

Buyer: https://backpack.tf/profiles/76561198219836673#!/compare/1497830400/1497916800

Sellers:

10 keys:

http://www.tf2outpost.com/trade/29304327 - young

12 keys:

http://i.imgur.com/Twvjaj6.png - young

Sales at:

8

9 (5x)

10

11

13

Buyers at 8. Excluding 8.

Sale at 13 looks like an outlier. Excluding 13.

Sales at 9 are far too common to be ignored.

Going with 9-11.