Sale 1:

Sold for 15 keys + 15 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1524528000/1524614400

Buyer: https://backpack.tf/profiles/76561198196545999#!/compare/1509667200/1524614400

Untraceable, but bot.tf confirms: https://i.imgur.com/A8ckrm6.png

X = 16

Sale 2:

Sold for 16 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1523664000/1523750400

Buyer: https://backpack.tf/profiles/76561198066584736#!/compare/1523664000/1523750400

Bot.tf confirms: https://i.imgur.com/A8ckrm6.png

X = 16

Sale 3:

Sold for 15 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198044130442#!/compare/1524614400/1524700800

Buyer: https://backpack.tf/profiles/76561198068946926#!/compare/1522281600/1525910400

X = 15

Sale 4:

Sold for 14 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1524787200/1524873600

Buyer: https://backpack.tf/profiles/76561198273700921#!/compare/1522454400/1525910400

Untraceable, but bot.tf confirms: https://i.imgur.com/A8ckrm6.png

X = 14

Sale 5:

Sold for 14 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1525046400/1525132800

Buyer: https://backpack.tf/profiles/76561198023662989#!/compare/1524700800/1525132800

Untraceable, (1 key missing) but bot.tf confirms: https://i.imgur.com/A8ckrm6.png

X = 14

Sale 6:

Sold for 14 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198044130442#!/compare/1524873600/1524960000

Buyer: https://backpack.tf/profiles/76561198066584736#!/compare/1524873600/1524960000

X = 14

Sale 7:

Sold for 13 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1525910400/1525996800

Buyer: https://backpack.tf/profiles/76561198040280241#!/compare/1525910400/1525996800

Bot.tf confirms: https://i.imgur.com/A8ckrm6.png

X = 13

Sale 8:

Sold for 13 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198334598690#!/compare/1525910400/1525996800

Buyer: https://backpack.tf/profiles/76561198115378991#!/compare/1525996800/1526169600

Bot.tf confirms: https://i.imgur.com/A8ckrm6.png

X = 13

Sale 9:

Sold with a key for Electric Hat Protector Universal Translator

No outpost

Seller: https://backpack.tf/profiles/76561198067775866#!/compare/1524009600/1524182400

Buyer: https://backpack.tf/profiles/76561198200746046#!/compare/1524096000/1524182400

Will most likely fall into the range.

Sale 10:

Sold for $43.96 on marketplace.tf

https://marketplace.tf/items/1118;5;u3007

Quite clearly an outlier, excluding.

Sellers:

14 keys:

https://i.imgur.com/oOk1Gsk.png

https://backpack.tf/item/2846983912

17 keys:

https://i.imgur.com/nI17Wne.png

https://backpack.tf/item/2850789821

Offers:

http://www.tf2outpost.com/trade/29613877

Sales at:

13 (2x)

14 (3x)

15

16 (2x)

Excluded the bot.tf purchases, as they are quickbuys: https://i.imgur.com/A8ckrm6.png

Going with 13-16.