Sale 1:

Sold for Max's head (~19)

Seller: https://backpack.tf/profiles/76561198054758757#!/compare/1523750400/1523836800

Buyer: https://backpack.tf/profiles/76561198103701614#!/compare/1523836800/1523923200

X = 19

Sale 2:

Sold for 18 keys

Seller: https://backpack.tf/profiles/76561198006544512#!/compare/1518134400/1518220800

Buyer: https://backpack.tf/profiles/76561198112011757#!/compare/1518134400/1518220800

X = 18

Sale 3:

Sold for 18 keys + 14 ref

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1524096000/1524182400

Buyer: https://backpack.tf/profiles/76561198170046169#!/compare/1524182400/1524268800

Bot.tf confirms: https://i.imgur.com/RDntmBv.png

X = 18

Sale 4:

Sold for 18 keys

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1518912000/1518998400

Buyer: https://backpack.tf/profiles/76561198173092790#!/compare/1518998400/1519084800

X = 18

Sale 5:

Sold to a quickbuyer? for 16 keys

Seller: https://backpack.tf/profiles/76561198168319836#!/compare/1521072000/1521331200

Buyer: https://backpack.tf/profiles/76561198050194207#!/compare/1521244800/1521331200

X = 16

Sale 6:

Sold for Grail Pool Party + Haunted Freedom Feathers

No outpost

Seller: https://backpack.tf/profiles/76561198336156500#!/compare/1523836800/1523923200

Buyer: https://backpack.tf/profiles/76561198271895036#!/compare/1523836800/1523923200

Not using this sale without a B/O.

Sellers:

~18.4 keys:

https://i.imgur.com/DR1GYX7.png

https://backpack.tf/item/1333834860

~18.5 keys:

https://i.imgur.com/hxOP9MY.png

https://backpack.tf/item/6390102024

20 keys:

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

https://backpack.tf/item/1597933802

Sales at:

16

18 (3x)

19

https://i.imgur.com/RDntmBv.png - Given the sale at 16 probably went to a quickbuyer, and the bot.tf purchase at ~17 keys, it should probably be fine to exclude 16.

Going with 18-19.