4 of these sales are mine :P

Sale 1:

Sold for Kill-a-Watt Tough Guy’s Toque (Mini at 18 below) + 1 key

No outpost

Seller: http://backpack.tf/profiles/76561198043060136#!/compare/1474502400/1474934400

Buyer: http://backpack.tf/profiles/76561198194759873#!/compare/1474934400/1475020800

X = 19

Sale 2:

Sold for 18 keys

http://www.tf2outpost.com/trade/28503943 - B/O of 18

Seller: http://backpack.tf/profiles/76561198021055079#!/compare/1474761600/1474848000

Buyer: http://backpack.tf/profiles/76561198045656784#!/compare/1464652800/1474848000

http://i.imgur.com/sxZ1wlB.png

http://i.imgur.com/DAzPw9p.jpg

X = 18

Sale 3:

Sold for 18 keys

Seller: http://backpack.tf/profiles/76561198021055079#!/compare/1473292800/1473379200

Buyer: http://backpack.tf/profiles/76561198088642885#!/compare/1471392000/1473379200

http://i.imgur.com/kTxY5O4.png

http://i.imgur.com/UwTqQ8W.png

X = 18

Sale 4:

Sold for 17 keys + 21 ref

http://www.tf2outpost.com/trade/28493405 - B/O of 18

Seller: http://backpack.tf/profiles/76561198021055079#!/compare/1474243200/1474329600

Buyer: http://backpack.tf/profiles/76561198043060136#!/compare/1474329600/1474502400

http://i.imgur.com/acqLmp5.png

http://i.imgur.com/3u2uLTO.png

X = 18

Sale 5:

Sold for Disco Beat Down Buckaroo’s Hat (20) + ~1.5 keys in items

http://www.tf2outpost.com/trade/28468252 - B/O of 18

Seller: http://backpack.tf/profiles/76561198021055079#!/compare/1473465600/1473552000

Buyer: http://backpack.tf/profiles/76561198049369645#!/compare/1473465600/1473552000

http://i.imgur.com/SW7xT3t.png

http://i.imgur.com/LI9JRw0.jpg

http://i.imgur.com/O3M8SAF.jpg

X = 18 (Capped by B/O)

Sale 6:

Sold with 1 key for Pro. KS Vintage Lugermorph (8) + Pro. KS Blood Botkiller Scattergun (12.5)

http://www.tf2outpost.com/trade/28440281 - B/O of 18

Seller: http://backpack.tf/profiles/76561198111914012#!/compare/1471996800/1472083200

Buyer: http://backpack.tf/profiles/76561198271925295#!/compare/1471996800/1472083200

X = 18

Sale 7:

Sold for Strange Spec. KS Original (7) + Strange Spec. KS Night Terror Scattergun (Field-Tested) (4) + 2 keys + Lime Crone’s Dome (~0.5)

No outpost

Seller: http://backpack.tf/profiles/76561198067312802#!/compare/1471910400/1472342400

Buyer: http://backpack.tf/profiles/76561198111914012#!/compare/1471910400/1471996800

X = 14

Sale 8:

Sold for Purple Confetti Old Guadalajara + 12 ref

No outpost

Seller: http://backpack.tf/profiles/76561198078701192#!/compare/1471305600/1471478400

Buyer: http://backpack.tf/profiles/76561198209627208#!/compare/1471478400/1471564800

Guadalajara looks quite overpriced, would probably be better to update it with this sale instead.

Kill-a-Watt Toque Mini:

Sale 1:

Sold for Pro. KS Rocket Launcher Kit (~20)

No outpost

Seller: http://backpack.tf/profiles/76561198090956917#!/compare/1474848000/1474934400

Buyer: http://backpack.tf/profiles/76561198121778217#!/compare/1474848000/1474934400

X = 20

Sale 2:

Sold for 16 keys

http://www.tf2outpost.com/trade/28354164 - B/O of 16

Seller: http://backpack.tf/profiles/76561198042938501#!/compare/1472342400/1472428800

Buyer: http://backpack.tf/profiles/76561198022105864#!/compare/1472428800/1472515200

X = 16

Going with 16-20 (18) for the Toque

Sellers:

18 keys:

http://i.imgur.com/WDtm2c2.png - 6 days

http://backpack.tf/u/76561198090956917

http://www.tf2outpost.com/trade/28320209 - 2 months (poorly bumped, with only 53 views at time of suggestion)

http://backpack.tf/profiles/76561198082467289

19 keys:

http://i.imgur.com/hMWt8yX.png - 2 months

http://www.tf2outpost.com/trade/28437560

http://backpack.tf/u/76561198006544512

Sales at:

14

18 (5x)

19

My purchases were all 16 keys pure each via my buy order (which is currently closed due to lack of keys)

Sale at 14 is an obvious outlier

Excluding 19 due to the 2 month old seller at 19 keys

Going with 18 flat