Using sales from April onwards.

Sale 1:

Sold for $19.99 on Marketplace.tf

https://marketplace.tf/items/30843;5;u3004

19.99/1.96 = ~10.2

X = 10

Sale 2:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198024867201#!/compare/1522540800/1522627200

Buyer: https://backpack.tf/profiles/76561198220465784#!/compare/1522540800/1524614400

X = 9

Sale 3:

Sold for Balloonihoodie (~5) + ~1.5 keys in items

http://www.tf2outpost.com/trade/29599514 - B/O of 9

Seller: https://backpack.tf/profiles/76561198257805373#!/compare/1522627200/1523836800

Buyer: https://backpack.tf/profiles/76561197999173629#!/compare/1523577600/1523664000

X = 6.5

Sale 4:

Sold for 6 keys + small amount of metal

Seller: https://backpack.tf/profiles/76561198369204942#!/compare/1523664000/1523750400

Buyer: https://backpack.tf/profiles/76561198273180640#!/compare/1523750400/1523923200

X = 6

Sale 5:

Sold for cheap skins

Seller: https://backpack.tf/profiles/76561198131568306#!/compare/1524614400/1525132800

Buyer: https://backpack.tf/profiles/76561198125021096#!/compare/1525132800/1525824000

X = 0

Sellers:

7 keys:

https://i.imgur.com/0uG2kj7.png

https://backpack.tf/item/6469757656

https://backpack.tf/item/6354789870

10 keys:

https://i.imgur.com/gMcrif0.png

https://backpack.tf/item/6549730864

12 keys:

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

https://backpack.tf/item/6252988648

17 keys:

https://i.imgur.com/O3mlHw2.png

https://backpack.tf/item/6412594871

Offers:

http://www.tf2outpost.com/trade/29594116

Sales at:

0

6

6.5

9

10

0 is an outlier.

9 and 10 are outliers, given the cheaper sellers. Excluding 9 and 10.

Going with 6-6.5.