Sale 1:

Sold for 13 keys

No outpost

Seller: http://backpack.tf/profiles/76561198066195760#!/compare/1477958400/1478044800

Buyer: http://backpack.tf/profiles/76561198051263429#!/compare/1478044800/1478131200

X = 13

Sale 2:

Sold for 11 keys + 11 ref (0.5, keys were 22 ref at the time)

http://www.tf2outpost.com/trade/28322521 - B/O of 12

Seller: http://backpack.tf/profiles/76561198057422775#!/compare/1472428800/1472515200

Buyer: http://backpack.tf/profiles/76561198084683310#!/compare/1472256000/1472515200

X = 12

Sale 3:

Sold for 11 keys

No outpost

Seller: http://backpack.tf/profiles/76561198064573218#!/compare/1474329600/1474416000

Buyer: http://backpack.tf/profiles/76561198190727030#!/compare/1474156800/1474416000

X = 11

Sale 4:

Sold for Strange Factory New Corsair Scattergun (6) + 4 keys + ~1 key in items

http://www.tf2outpost.com/trade/28442950 - No B/O

Seller: http://backpack.tf/profiles/76561198118957055#!/compare/1472688000/1472860800

Buyer: http://backpack.tf/profiles/76561198085879041#!/compare/1472774400/1472860800

X = 11

Sale 5:

Sold for 80 ref (~3.5) + 2 keys in junk…

Seller: http://backpack.tf/profiles/76561198120482330#!/compare/1473552000/1474070400

Buyer: http://backpack.tf/profiles/76561198057715695#!/compare/1473984000/1474070400

X = 6

Sales at:

13

12

11 (2x)

6

No current sellers

Excluding 6 for obvious reasons…

Going with 11-13