Sale 1:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1516665600/1516752000

Buyer: https://backpack.tf/profiles/76561198159197658#!/compare/1516752000/1516924800

X = 11

Sale 2:

Sold for 8 keys + 6 ref to a buyer

Seller: https://backpack.tf/profiles/76561198155320774#!/compare/1516752000/1516924800

Buyer: https://backpack.tf/profiles/76561198344933095#!/compare/1516838400/1516924800

X = 8

Sale 3:

Bulk for Terror-Watt Tough Stuff Muffs

Seller: https://backpack.tf/profiles/76561198243458980#!/compare/1516492800/1516665600

Buyer: https://backpack.tf/profiles/76561198170069027#!/compare/1516406400/1516665600

Sellers:

~8.6 keys:

https://i.imgur.com/2myZyGf.png - 8 days

https://backpack.tf/item/3670951079

9 keys:

https://i.imgur.com/swzRqzz.png - 1 day

https://backpack.tf/item/3592167154

11 keys:

https://i.imgur.com/gAV0R34.png - 11 days

https://backpack.tf/item/2985021857

12 keys:

https://i.imgur.com/iUh3mnj.png - 1 month

https://backpack.tf/item/2751053690

18 keys:

https://i.imgur.com/MGL5Faj.png - 4 days

https://backpack.tf/item/4195133148

24 keys:

https://i.imgur.com/L3kFNa6.png - 7 days

https://backpack.tf/item/2823939542

Sales at:

8

11

8 may have gone to a quickbuyer, but given the seller at 8.6, it should be fair to include it.

Going with 8-11.

Unable to check marketplace.tf right now, as the site is down.