Adding three sales to the previous accepted suggestion:

Sale 1:

Sold for Max’s Head (18 at the time)

No outpost

Seller: http://backpack.tf/profiles/76561198074924284#!/compare/1481068800/1481155200

Buyer: http://backpack.tf/profiles/76561198160531375#!/compare/1481068800/1481155200

X = 18

Sale 2:

Sold for 18 keys

http://www.tf2outpost.com/trade/28617474 - B/O of 20

Seller: http://backpack.tf/profiles/76561198051069962#!/compare/1480032000/1480118400

Buyer: http://backpack.tf/profiles/76561198051069962#!/compare/1480032000/1480118400

http://imgur.com/a/GkTrN

X = 18

Sale 3:

Sold for Collector’s Degreaser (9)

Seller: http://backpack.tf/profiles/76561198280729971#!/compare/1485388800/1485475200

Buyer: http://backpack.tf/profiles/76561198044684270#!/compare/1485302400/1485388800

X = 9

From the last suggestion: (Credit to Blaze.xTH3RM4Lx)

http://backpack.tf/suggestion/589373d90e2cad5fa639693b

Sold:

https://puu.sh/tLeA5/5c74df24d5.png

https://puu.sh/tLeBi/5ee6dc9f09.png - sold for 15 keys

Credits to Woifilicious

http://backpack.tf/profiles/76561198102036282#!/compare/1484524800/1485648000 - sold for 13 keys (quicksold)

Going with 15-18