Sale 1:

Sold for 6 keys + ~15 keys in items

No outpost

Seller: https://backpack.tf/profiles/76561198274139380#!/compare/1506643200/1506729600

Buyer: https://backpack.tf/profiles/76561198086719211#!/compare/1506297600/1506729600

X = 21

Sale 2:

Sold for 19 keys + cheap item

Seller: https://backpack.tf/profiles/76561198057671085#!/compare/1504915200/1505779200

Buyer: https://backpack.tf/profiles/76561198161860788#!/compare/1505606400/1505779200

X = 19

Sale 3:

Sold for Australium Knife (16)

No outpost

Seller: https://backpack.tf/profiles/76561197974364849#!/compare/1505952000/1506124800

Buyer: https://backpack.tf/profiles/76561198147868033#!/compare/1505865600/1506124800

X = 16

Sale 4:

Bulk

Seller: https://backpack.tf/profiles/76561198257905663#!/compare/1505692800/1505779200

Buyer: https://backpack.tf/profiles/76561198182213262#!/compare/1505692800/1505779200

Sale 5:

Bulk

Seller: https://backpack.tf/profiles/76561198182213262#!/compare/1505779200/1505865600

Buyer: https://backpack.tf/profiles/76561198159593026#!/compare/1505779200/1505865600

Sale 6:

Sold for Tesla Coil Katuysha

Seller: https://backpack.tf/profiles/76561198159593026#!/compare/1505779200/1505865600

Buyer: https://backpack.tf/profiles/76561197974364849#!/compare/1505779200/1505865600

Can be used on the Katuysha

Sellers:

21 keys:

https://i.imgur.com/w9p5XBI.jpg - 3 days

Sales at:

16

19

21

16 seems kinda low, and it sold in ~3 days. Excluding 16.

Going with 19-21.