Sales 1 & 2:

Sold for 12 keys, then sold back to the bot for 8 keys + 19 ref on the same day. Both sales can be seen with these links.

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1515369600/1515456000

Buyer: https://backpack.tf/profiles/76561198141689040#!/compare/1515369600/1515456000

X = 9 & 12

Sale 3:

Sold to a quickbuying bot for 9 keys

Seller: https://backpack.tf/profiles/76561198086611203#!/compare/1514592000/1514678400

Buyer: https://backpack.tf/profiles/76561198344933095#!/compare/1514592000/1514678400

X = 9

Sale 4:

Sold for 9 keys + 5 ref

Seller: https://backpack.tf/profiles/76561198128403236#!/compare/1514419200/1514505600

Buyer: https://backpack.tf/profiles/76561198040922869#!/compare/1514419200/1514505600

X = 9

Sale 5:

Sold for 8 keys to a quickbuyer

Seller: https://backpack.tf/profiles/76561198170552425#!/compare/1516320000/1516406400

Buyer: https://backpack.tf/profiles/76561198048851745#!/compare/1516320000/1516406400

X = 8

Sale 6:

Sold for 7 keys + ~1 key in items

Seller: https://backpack.tf/profiles/76561198275581475#!/compare/1516406400/1516579200

Buyer: https://backpack.tf/profiles/76561198400251476#!/compare/1516492800/1516579200

X = 8

Sale 7:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198016078628#!/compare/1516320000/1516492800

Buyer: https://backpack.tf/profiles/76561198170552425#!/compare/1516147200/1516320000

X = 7

Sellers:

~8.4 keys:

https://i.imgur.com/dWhW81I.png - 17 hours

https://backpack.tf/item/6324498385

10 keys:

https://i.imgur.com/Nh8ppJ2.png - 8 hours

https://backpack.tf/item/2906920859

12 keys:

https://i.imgur.com/exXl5Dp.png - 3 days

https://backpack.tf/item/2805476566

18 keys:

https://i.imgur.com/mUB8lJS.png - 1 month

https://backpack.tf/item/4868068749

Sales at:

7

8 (2x)

9 (3x)

12

excluding 7 and 12, low and high outliers.

8-9.