Sale 1:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198155320889#!/compare/1516233600/1516320000

Buyer: https://backpack.tf/profiles/76561198117923240#!/compare/1516233600/1516406400

X = 12

Sale 2:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198004383856#!/compare/1515888000/1515974400

Buyer: https://backpack.tf/profiles/76561198098480978#!/compare/1515974400/1516060800

X = 11

Sale 3:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198312663253#!/compare/1514851200/1514937600

Buyer: https://backpack.tf/profiles/76561198059956339#!/compare/1514851200/1515024000

X = 11

Sale 4:

Sold for 10 keys + 19 ref to a quickbuyer

Seller: https://backpack.tf/profiles/76561198114402400#!/compare/1514678400/1514764800

Buyer: https://backpack.tf/profiles/76561198164651434#!/compare/1514678400/1514764800

X = 11

Sale 5:

Sold for 11 keys to a bot/service thing? human

Seller: https://backpack.tf/profiles/76561198156255538#!/compare/1516233600/1516320000

Buyer: https://backpack.tf/profiles/76561198307759744#!/compare/1516320000/1516406400

https://backpack.tf/item/2766924018

X = 11

Sale 6:

Sold to a quickbuyer for 10 keys

Seller: https://backpack.tf/profiles/76561198088690820#!/compare/1514505600/1514764800

Buyer: https://backpack.tf/profiles/76561198114402400#!/compare/1514592000/1514764800

X = 10

Sale 7:

Bulk

Seller: https://backpack.tf/profiles/76561198314160142#!/compare/1516320000/1516406400

Buyer: https://backpack.tf/profiles/76561198085269574#!/compare/1516320000/1516406400

Sale 8:

Bulk for Infernal Smoke Proletariat

Seller: https://backpack.tf/profiles/76561198070385360#!/compare/1515024000/1515283200

Buyer: https://backpack.tf/profiles/76561198156255538#!/compare/1515196800/1515283200

Sellers:

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

10 keys:

https://backpack.tf/item/4492915789 - 3 days

12 keys:

https://backpack.tf/item/3823985538 - 2 days

https://backpack.tf/item/2701126609 - 2 days

13 keys:

https://backpack.tf/item/2700487092 - 29 days

25 keys:

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

Sales at:

10

11 (4x)

12

Seller at 10 tells us that the sales that went to quickbuyers shouldn’t be excluded.

Going with 10-12.