Sale 1:

Sold for 20 keys

Seller: https://backpack.tf/profiles/76561198157183121#!/compare/1507075200/1507161600

Buyer: https://backpack.tf/profiles/76561198148833495#!/compare/1507075200/1507161600

X = 20

Sale 2:

Sold for Spec. KS Grenade Launcher (13) + ~6 keys in items

No outpost

Seller: https://backpack.tf/profiles/76561198285293881#!/compare/1507766400/1507852800

Buyer: https://backpack.tf/profiles/76561198153789411#!/compare/1507766400/1507852800

X = 19

Sale 3:

Sold for 19 keys

Seller: https://backpack.tf/profiles/76561197994697258#!/compare/1504483200/1504569600

Buyer: https://backpack.tf/profiles/76561198175772830#!/compare/1504483200/1504569600

X = 19

Sale 4:

Quicksold for 17 keys + 18 ref

Seller: https://backpack.tf/profiles/76561197963500354#!/compare/1505088000/1505174400

Buyer: https://backpack.tf/profiles/76561198157183121#!/compare/1505088000/1505174400

X = 18

Sale 5:

Quicksold for 16 keys

Seller: https://backpack.tf/profiles/76561198119927852#!/compare/1510876800/1510963200

Buyer: https://backpack.tf/profiles/76561198049866989#!/compare/1510012800/1510963200

X = 16

Sale 6:

Sold for Well Worn Isotope Blitzkrieg SMG

Seller: https://backpack.tf/profiles/76561198148833495#!/compare/1507593600/1507766400

Buyer: https://backpack.tf/profiles/76561198285293881#!/compare/1507680000/1507766400

Sale 7:

Bulk for Isotope Minimal Wear Carpet Bomber Stickybomb Launcher

Seller: https://backpack.tf/profiles/76561198153789411#!/compare/1508112000/1508198400

Buyer: https://backpack.tf/profiles/76561198240745690#!/compare/1508112000/1508198400

Sale 8:

Bulk for Haunted Ghosts Prince Tavish’s Crown

Seller: https://backpack.tf/profiles/76561198240745690#!/compare/1509840000/1509926400

Buyer: https://backpack.tf/profiles/76561198030050182#!/compare/1509840000/1509926400 (hats transferred to/from their bot)

Sale 9:

Bulk

Seller: https://backpack.tf/profiles/76561198158206236#!/compare/1510876800/1510963200

Buyer: https://backpack.tf/profiles/76561198119927852#!/compare/1510876800/1510963200

Sale 10:

Bulk for Hot Well Worn Night Terror Scattergun

Seller: https://backpack.tf/profiles/76561198147833204#!/compare/1506816000/1507680000

Buyer: https://backpack.tf/profiles/76561198076020691#!/compare/1506729600/1506902400

Sale 11:

Sold with ~4 keys in pure/items for Planets Bubble Pipe

Seller: https://backpack.tf/profiles/76561198071754044#!/compare/1509840000/1510012800

Buyer: https://backpack.tf/profiles/76561198158206236#!/compare/1509840000/1510012800

Should be used on the pipe.

Sellers:

17 keys:

https://i.imgur.com/EQ0iq7J.png - 23 days

19 keys:

https://i.imgur.com/a9uFKT5.png - 4 days

21 keys:

http://www.tf2outpost.com/trade/29453063 - 5 days

Sales at:

16

18

19 (2x)

20

16 quicksold. Excluding 16.

18 should not be excluded, as the seller at 17 is 23 days old.

Going with 17-20.