Sale 1:

Sold for 10 keys (ID's match)

No outpost

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

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

X = 10

Sale 2:

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 3:

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 4:

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 5:

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 6:

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 7:

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 (4x)

10

Buyers at 8. Excluding 8.

Sales at 9 are far too common to be ignored.

Going with 9-10.