Sale 1:

Sold for 18 keys

https://imgur.com/a/bvRab

https://backpack.tf/item/2375118741

X = 18

Sale 2:

Sold for 17 keys + ~1 key in items

No Outpost

Seller: https://backpack.tf/profiles/76561198143417110#!/compare/1504569600/1504656000

Buyer: https://backpack.tf/profiles/76561198145961558#!/compare/1504569600/1504656000

X = 18

Sale 3:

Sold for 18 keys

No outpost

Seller: https://backpack.tf/profiles/76561198009458322#!/compare/1501459200/1501545600

Buyer: https://backpack.tf/profiles/76561198098218334#!/compare/1501459200/1501545600

X = 18

Sale 4:

Sold for $33 on Marketplace.tf

https://marketplace.tf/items/95;5;u61

https://backpack.tf/item/3001725207 (Might not be this one)

33/1.91 = ~17.3

X = 17

Sale 5:

Sold for 17 keys

http://www.tf2outpost.com/trade/29369788 - B/O of 17

Seller: https://backpack.tf/profiles/76561198021055079#!/compare/1505606400/1505692800

Buyer: https://backpack.tf/profiles/76561198373950141#!/compare/1505606400/1505692800

X = 17

Sale 6:

Quicksold for 13 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198094274080#!/compare/1504396800/1504569600

Buyer: https://backpack.tf/profiles/76561198122604075#!/compare/1504483200/1504569600

X = 13

Sale 7:

Bulk for Planets KE

Seller: https://backpack.tf/profiles/76561198145961558#!/compare/1505520000/1505606400

Buyer: https://backpack.tf/profiles/76561198172314406#!/compare/1505520000/1505606400

Sale 8:

Bulk for Haunted Ghosts Safe’n’Sound

Seller: https://backpack.tf/profiles/76561198172314406#!/compare/1505865600/1505952000

Buyer: https://backpack.tf/profiles/76561198067557593#!/compare/1505865600/1505952000

Sale 9:

Bulk for Ether Trail Smokey Sombrero

Seller: https://backpack.tf/profiles/76561198098218334#!/compare/1501977600/1502064000

Buyer: https://backpack.tf/profiles/76561198198392589#!/compare/1501545600/1502064000

Sale 10:

Bulk for Circling Heart Western Wear

Seller: https://backpack.tf/profiles/76561198078283443#!/compare/1504396800/1504569600

Buyer: https://backpack.tf/profiles/76561198094274080#!/compare/1504396800/1504569600

Sales at:

13

17 (2x)

18 (3x)

13 quicksold, excluding 13.

Going with 17-18.