Sale 1:

Sold for Pro. KS Aussie Axtinguisher (6.5) + Pro. KS Strange Rocket Launcher (13.5)

http://www.tf2outpost.com/trade/29385053 - B/O of 19

Seller: https://backpack.tf/profiles/76561198218497692#!/compare/1506729600/1506816000

Buyer: https://backpack.tf/profiles/76561198068090261#!/compare/1506643200/1506816000

X = 19 (Capped by B/O)

Sale 2:

Sold for Miami Nights Nanobalaclava + 8 keys

http://www.tf2outpost.com/trade/29402294 - B/O of 15.5

Seller: https://backpack.tf/profiles/76561198153806405#!/compare/1508976000/1509148800

Buyer: https://backpack.tf/profiles/76561198280359073#!/compare/1509062400/1509148800

Easily caps.

X = 16 (Capped by B/O, rounded)

Sale 3:

Sold for Strange Pro. KS Machina (13) + 2 EOTL keys + Botkiller wrench

No outpost

Seller: https://backpack.tf/profiles/76561198068090261#!/compare/1506816000/1506902400

Buyer: https://backpack.tf/profiles/76561198145843391#!/compare/1506729600/1506816000

X = 15

Sale 4:

Sold with some ref for Stormy Storm Jumper’s Jeepcap

Seller: https://backpack.tf/profiles/76561198224899376#!/compare/1506297600/1506384000

Buyer: https://backpack.tf/profiles/76561198182213262#!/compare/1506297600/1506384000

Sale 5:

Bulk for Burning Buckaroo’s

Seller: https://backpack.tf/profiles/76561198182213262#!/compare/1506643200/1506729600

Buyer: https://backpack.tf/profiles/76561198200169799#!/compare/1506643200/1506729600

Sale 6:

Bulk for Stormy Storm Cold Killer

Seller: https://backpack.tf/profiles/76561198200169799#!/compare/1506643200/1506729600

Buyer: https://backpack.tf/profiles/76561198218497692#!/compare/1506643200/1506729600

Sale 7:

Bulk for Green Confetti Pomade Prince

Seller: https://backpack.tf/profiles/76561198147449497#!/compare/1508198400/1508284800

Buyer: https://backpack.tf/profiles/76561198116732510#!/compare/1508112000/1508198400

Sale 8:

Bulk for Silver Cyclone Balloonibouncer

Seller: https://backpack.tf/profiles/76561198116732510#!/compare/1509667200/1509753600

Buyer: https://backpack.tf/profiles/76561198211995121#!/compare/1509753600/1509840000

Sale 9:

Bulk for Nebula Universal Translator

Seller: https://backpack.tf/profiles/76561198211995121#!/compare/1509840000/1509926400

Buyer: https://backpack.tf/profiles/76561197994697258#!/compare/1509753600/1509840000

Sellers:

17 keys:

https://i.imgur.com/XtFlQj3.png – 5 days, lowered from 20

https://backpack.tf/item/2367473893

https://i.imgur.com/LJ0yvYC.png - 6 days

https://backpack.tf/item/2004613869

18 keys:

https://i.imgur.com/03S9tjp.png - 3 days

https://backpack.tf/item/2869865129

19 keys:

https://i.imgur.com/rEAtyQu.png - 9 days, definitely lowered/re-listed

https://backpack.tf/item/2838757377

Sales at:

15

16

19

19 looks like an outlier, and there is a fair amount of cheaper sellers. Excluding 19.

Going with 15-16.