Using last 2 months.

Sales 1 & 2:

Sold for $19.49 and $15 on Marketplace.tf

https://marketplace.tf/items/1182;5;u3007

19.49/1.96 = ~9.94

15/1.97 = ~7.61

X = 10

X = 7.5

Sale 3:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198097271466#!/compare/1521158400/1521244800

Buyer: https://backpack.tf/profiles/76561198148242311#!/compare/1521158400/1521244800

X = 7

Sale 4:

Sold for 6 keys, most likely to a quickbuyer

Seller: https://backpack.tf/profiles/76561198065157321#!/compare/1516060800/1520812800

Buyer: https://backpack.tf/profiles/76561198000168096#!/compare/1520726400/1520812800

This sale is pretty old, should be fine to ignore the quickbuyer part, given the current seller at 6, and the other sale at 6 here.

X = 6

Sale 5:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198048851745#!/compare/1521849600/1521936000

Buyer: https://backpack.tf/profiles/76561198074161948#!/compare/1521936000/1522022400

X = 6

Sale 6:

Sold with key + Spec. KS Original (~1) for SGG Fresh Brewed Victory

Seller: https://backpack.tf/profiles/76561198148242311#!/compare/1522281600/1522368000

Buyer: https://backpack.tf/profiles/76561198085628805#!/compare/1522281600/1522368000

Should be used on the Fresh Brewed Victory.

Sale 7:

Bulk for Nebula Spacemann

Seller: https://backpack.tf/profiles/76561198085628805#!/compare/1522368000/1522540800

Buyer: https://backpack.tf/profiles/76561198068789249#!/compare/1522368000/1522540800

Sale 8:

Bulk

Seller: https://backpack.tf/profiles/76561198074161948#!/compare/1522108800/1522281600

Buyer: https://backpack.tf/profiles/76561198115699456#!/compare/1522108800/1522281600

Sellers:

6 keys:

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

https://backpack.tf/item/6257633959

7 keys:

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

https://backpack.tf/item/6404169501

https://backpack.tf/item/6459205366

https://backpack.tf/item/6440930456

9 keys:

https://i.imgur.com/NsQXo7j.png

https://backpack.tf/item/6473571184

https://backpack.tf/item/6391025182

Sales at:

6 (2x)

7

7.5

10

Gonna exclude 7.5 (and 10), given the 3 sellers for cheaper.

Going with 6-7.