Sale 1:

Sold for 20 keys (ID’s match)

http://www.tf2outpost.com/trade/29367898 - B/O of 20

Seller: https://backpack.tf/profiles/76561198136180916#!/compare/1506556800/1506729600

Buyer: https://backpack.tf/profiles/76561198199355251#!/compare/1504915200/1506729600

X = 20

Sale 2:

Quicksold for 17 keys

Seller: https://backpack.tf/profiles/76561198117379982#!/compare/1505174400/1505260800

Buyer: https://backpack.tf/profiles/76561198136180916#!/compare/1504915200/1505260800

X = 17

Sale 3:

Quicksold for 17 keys

Seller: https://backpack.tf/profiles/76561198236788328#!/compare/1510963200/1511049600

Buyer: https://backpack.tf/profiles/76561198116745207#!/compare/1510963200/1511049600

X = 17

Sale 4:

Quicksold for 17 keys + 5 ref

Seller: https://backpack.tf/profiles/76561198116745207#!/compare/1511049600/1511136000

Buyer: https://backpack.tf/profiles/76561198160120451#!/compare/1511049600/1511136000

X = 17

Sale 5:

Quicksold for 17 keys + 5 ref

Seller: https://backpack.tf/profiles/76561198160120451#!/compare/1511049600/1511136000

Buyer: https://backpack.tf/profiles/76561198156680751#!/compare/1511049600/1511136000

x = 17

Sale 6:

Quicksold for 16 keys

Seller: https://backpack.tf/profiles/76561198346118738#!/compare/1510272000/1510358400

Buyer: https://backpack.tf/profiles/76561198015384426#!/compare/1510272000/1510358400

X = 16

Sale 7:

Bulk for Peace Sign Chill Chullo

Seller: https://backpack.tf/profiles/76561198147530243#!/compare/1510185600/1510272000

Buyer: https://backpack.tf/profiles/76561198346118738#!/compare/1510185600/1510272000

Sale 8:

Bulk for pure

Seller: https://backpack.tf/profiles/76561198015384426#!/compare/1510704000/1510790400

Buyer: https://backpack.tf/profiles/76561198236788328#!/compare/1510790400/1510876800

Sellers:

18 keys:

https://i.imgur.com/S3fG6ND.png - young

24 keys:

https://i.imgur.com/UJPBQ9G.png - ~1 year

Sales at:

16

17 (4x)

20

Sales at 17 are way too common to be excluded, even if all of them did sell in about a day. Will exclude 16, however.

Going with 17-20.