Sale 1:

Sold for Miami Nights Scotsman’s Stove Pipe (20.5) + 3 keys

No outpost

Seller: https://backpack.tf/profiles/76561198083800515#!/compare/1502150400/1502236800

Buyer: https://backpack.tf/profiles/76561198213963716#!/compare/1502150400/1502323200

X = 24

Sale 2:

Sold for Vivid Plasma Respectless Robo Glove (Suggestion at 20.5)

No outpost

Seller: https://backpack.tf/profiles/76561198382076026#!/compare/1501286400/1501372800

Buyer: https://backpack.tf/profiles/76561198083800515#!/compare/1501200000/1501286400

X = 21

Sale 3:

Sold for Aces High Executioner (19) + ~2 keys in items

No outpost

Seller: https://backpack.tf/profiles/76561198272595351#!/compare/1503792000/1503964800

Buyer: https://backpack.tf/profiles/76561198147254211#!/compare/1503878400/1503964800

X = 21

Sale 4:

Sold for Stormy Storm Attendant (16.5) + > 2 keys in items

No outpost

Seller: https://backpack.tf/profiles/76561198230964868#!/compare/1503273600/1503360000

Buyer: https://backpack.tf/profiles/76561198134641756#!/compare/1503273600/1503360000

X = 19

Sale 5:

Sold for Smoking Noble (18)

No outpost

Seller: https://backpack.tf/profiles/76561198299474630#!/compare/1503705600/1503792000

Buyer: https://backpack.tf/profiles/76561198167908292#!/compare/1503705600/1503792000

X = 18

Sale 6:

Sold for Purple Confetti Safe’n’Sound (18) + Strange Dark Falkirk Helm (1)

http://www.tf2outpost.com/trade/29258350 - B/O of 17

Seller: https://backpack.tf/profiles/76561198033370726#!/compare/1499472000/1499558400

Buyer: https://backpack.tf/profiles/76561198068189558#!/compare/1499472000/1499558400

X = 17 (Capped by B/O)

Sale 7:

Sold for Dead Presidents Hat With No Name (21)

http://www.tf2outpost.com/trade/29283568 - B/O of 16

Seller: https://backpack.tf/profiles/76561198056793444#!/compare/1500595200/1500681600

Buyer: https://backpack.tf/profiles/76561198121802323#!/compare/1500595200/1500681600

X = 16 (Capped by B/O)

Sale 8:

Sold for 15 keys + 20 ref (~1)

No outpost

Seller: https://backpack.tf/profiles/76561197963500354#!/compare/1501200000/1501286400

Buyer: https://backpack.tf/profiles/76561198028063084#!/compare/1501200000/1501286400

X = 16

Sale 9:

Sold for 16 keys

http://www.tf2outpost.com/trade/29301238 - B/O of 16

Seller: https://backpack.tf/profiles/76561198028063084#!/compare/1501804800/1501891200

Buyer: https://backpack.tf/profiles/76561198187833759#!/compare/1501718400/1501891200

X = 16

Sale 10:

Sold for 16 keys (IDs match)

http://www.tf2outpost.com/trade/29328795 - B/O of 16

Seller: https://backpack.tf/profiles/76561198119634686#!/compare/1503446400/1503532800

Buyer: https://backpack.tf/profiles/76561198047593891#!/compare/1503273600/1503532800

X = 16

Sale 11:

Sold for 15 keys

Seller: https://backpack.tf/profiles/76561198187833759#!/compare/1502064000/1502150400

Buyer: https://backpack.tf/profiles/76561198292242424#!/compare/1501977600/1502064000

X = 15

Sale 12:

Sold for 15 keys

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1499904000/1499990400

Buyer: https://backpack.tf/profiles/76561198150295432#!/compare/1499817600/1499990400

X = 15

Sale 13:

Sold for 14 keys + strange part (~1)

http://www.tf2outpost.com/trade/29281872 - B/O of 15

Seller: https://backpack.tf/profiles/76561198154253412#!/compare/1500336000/1500422400

Buyer: https://backpack.tf/profiles/76561197991111981#!/compare/1500336000/1500422400

X = 15

Sale 14:

Quicksold for 15 keys

No outpost

Seller: https://backpack.tf/profiles/76561198337734785#!/compare/1502928000/1503014400

Buyer: https://backpack.tf/profiles/76561198044137579#!/compare/1502582400/1503014400

X = 15

Sale 15:

Sold for Slumber Nasty Norseman

No outpost

Seller: https://backpack.tf/profiles/76561198121802323#!/compare/1503187200/1503273600

Buyer: https://backpack.tf/profiles/76561198299474630#!/compare/1503014400/1503619200

Can be used on the Norseman

Sale 16:

Bulk for Terror-Watt Executioner

Seller: https://backpack.tf/profiles/76561198213963716#!/compare/1503187200/1503273600

Buyer: https://backpack.tf/profiles/76561198230964868#!/compare/1503187200/1503273600

Sale 17:

Bulk for Slumber Executioner

Seller: https://backpack.tf/profiles/76561198159262475#!/compare/1498953600/1499126400

Buyer: https://backpack.tf/profiles/76561198056793444#!/compare/1499040000/1499126400

Vivid Plasma Robo Glove:

https://backpack.tf/suggestion/59a7e84ccf6c75311946c4f4

Sellers:

19 keys:

http://i.imgur.com/nriDAY1.png - 1 day

http://i.imgur.com/XAJY73l.png - 4 day

20 keys:

http://i.imgur.com/2gZyysR.jpg - 4 days

http://i.imgur.com/BVCj06v.png - 4 months

Sales at:

15 (4x)

16 (4x)

17

18

19

21 (2x)

24

21 and 24 are too high, with all the sellers for cheaper, and the 4-month-old seller at 20. Excluding 21 and 24.

15 is low, due to buyers at 15:

http://i.imgur.com/LNtXP05.png

Excluding 15.

Going with 16-19.