Sale 1:

Sold for $35.99 on Marketplace.tf

https://marketplace.tf/items/400;5;u29

https://backpack.tf/item/497256145

35.99/1.91 = ~18.8

X = 19

Sale 2:

Sold for 17 keys

No outpost

Seller: https://backpack.tf/profiles/76561197988155310#!/compare/1505952000/1506038400

Buyer: https://backpack.tf/profiles/76561198038860439#!/compare/1505952000/1506038400

X = 17

Sale 3:

Sold for 16 keys

No outpost

Seller: https://backpack.tf/profiles/76561198296062531#!/compare/1504915200/1505001600

Buyer: https://backpack.tf/profiles/76561198099661869#!/compare/1504656000/1508025600

X = 16

Sale 4:

Sold for 15 keys

No outpost

Seller: https://backpack.tf/profiles/76561198045388088#!/compare/1502928000/1503532800

Buyer: https://backpack.tf/profiles/76561198177152782#!/compare/1503532800/1503619200

X = 15

Sale 5:

Sold in about a day for 14 keys + 9 ref

Seller: https://backpack.tf/profiles/76561198084126218#!/compare/1502064000/1502150400

Buyer: https://backpack.tf/profiles/76561198075350866#!/compare/1502064000/1502150400

X = 14

Sale 6:

Bulk for something

Seller: https://backpack.tf/profiles/76561198177152782#!/compare/1504569600/1504656000

Buyer: https://backpack.tf/profiles/76561198243530442#!/compare/1504569600/1504656000

Sale 7:

Bulk for Green Energy Capotain

Seller: https://backpack.tf/profiles/76561198132896081#!/compare/1501891200/1501977600

Buyer: https://backpack.tf/profiles/76561198079504588#!/compare/1501804800/1501977600

Sale 8:

Sold with ~4 keys in pure/items for Circling Heart Crocleather Slouch

Seller: https://backpack.tf/profiles/76561198061067156#!/compare/1504828800/1505520000

Buyer: https://backpack.tf/profiles/76561198199729741#!/compare/1504742400/1505088000

Should be used on the Croc.

Sellers:

18 keys:

https://i.imgur.com/TNzpHJ6.png

Sales at:

14

15

16

17

19

14 quicksold, excluding 14.

Going with 15-19.