Sale 1:

Sold for Logo Fez (Mini at 16 below) + less than 3.5 keys worth

http://www.tf2outpost.com/trade/29499058 - B/O of 20

Seller: https://backpack.tf/profiles/76561198194830086#!/compare/1515283200/1515888000

Buyer: https://backpack.tf/profiles/76561198212547641#!/compare/1515801600/1515888000

X = 19

Sale 2:

Sold for Stormy Bolted Bicorne (Suggestion at 19) + more than 2.5 keys worth

http://www.tf2outpost.com/trade/29515158 - B/O of 18

Seller: https://backpack.tf/profiles/76561198011671663#!/compare/1514937600/1515024000

Buyer: https://backpack.tf/profiles/76561198109871970#!/compare/1514937600/1515024000

X = 18 (Capped by B/O)

Sale :

Sold for Orbiting Fire Hustler's Hallmark (Mini at 12 below) + 4 keys

No outpost

Seller: https://backpack.tf/profiles/76561198388652048#!/compare/1515024000/1515456000

Buyer: https://backpack.tf/profiles/76561198121280601#!/compare/1515369600/1515456000

X = 16

Sale :

Sold for 15 keys

Seller: https://backpack.tf/profiles/76561198104055806#!/compare/1514592000/1514678400

Buyer: https://backpack.tf/profiles/76561198194830086#!/compare/1496448000/1514678400

X = 15

Sale :

Quicksold for 15 keys

Seller: https://backpack.tf/profiles/76561198251777504#!/compare/1514851200/1514937600

Buyer: https://backpack.tf/profiles/76561198011671663#!/compare/1514851200/1514937600

X = 15

Sale :

Quicksold for 14 keys

Seller: https://backpack.tf/profiles/76561198796804702#!/compare/1515369600/1515628800

Buyer: https://backpack.tf/profiles/76561198011671663#!/compare/1515542400/1515628800

X = 14

Sale :

Quicksold for some amount of keys

Seller: https://backpack.tf/profiles/76561198121280601#!/compare/1515542400/1515628800

Buyer: https://backpack.tf/profiles/76561198796804702#!/compare/1515369600/1515628800

Sale :

Sold for SGG Headcase + 5 keys + cheap items

Seller: https://backpack.tf/profiles/76561198216631153#!/compare/1514764800/1514851200

Buyer: https://backpack.tf/profiles/76561198251777504#!/compare/1514764800/1514851200

Given the sellers on the Headcase, this'll be around quickbuyer price. Excluding.

Stormy Bolted Bicorne:

https://backpack.tf/suggestion/5a70589344325a65b54934b3

Logo Fez Mini:

Sold with 2 keys for Purple Energy Handle (22)

http://webcache.googleusercontent.com/search?q=cache:0lkCQduhw8oJ:www.tf2outpost.com/trade/29503548+&cd=1&hl=en&ct=clnk&gl=au

https://i.imgur.com/Gyp1UT7.png - B/O of 16

Seller: https://backpack.tf/profiles/76561198032176942#!/compare/1514937600/1515024000

Buyer: https://backpack.tf/profiles/76561198050930436#!/compare/1514764800/1515024000

X = 16 (Capped by B/O)

Orbiting Fire Hustler's Hallmark Mini:

Sale 1:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198179707937#!/compare/1516924800/1517011200

BUyer: https://backpack.tf/profiles/76561198254703163#!/compare/1516924800/1517011200

X = 12

Sale 2:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198061440669#!/compare/1510704000/1510790400

Buyer: https://backpack.tf/profiles/76561198060583505#!/compare/1510790400/1511308800

X = 12

Sellers:

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

15 keys:

https://backpack.tf/item/4881147085 - 2 days

17 keys:

https://backpack.tf/item/6402973201 - 15 days

18 keys:

https://backpack.tf/item/2357051680 - a while

Sales at:

14

15 (2x)

16

18

19

14 quicksold, excluding 14.

15 quicksold once (at least), but the seller at 15 supports 15 being in the range.

Going with 15-19.