Sale 1:

Sold for 10 keys (IDs match)

Seller: https://backpack.tf/profiles/76561198190577279#!/compare/1520121600/1520294400

Buyer: https://backpack.tf/profiles/76561198430844884#!/compare/1520121600/1520208000

X = 10

Sale 2:

Sold for 10 keys + 5 ref

Seller: https://backpack.tf/profiles/76561198430844884#!/compare/1520812800/1520899200

Buyer: https://backpack.tf/profiles/76561198144099154#!/compare/1520812800/1520899200

X = 10

Sale 3:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198085169466#!/compare/1519776000/1520121600

Buyer: https://backpack.tf/profiles/76561198190577279#!/compare/1520035200/1520121600

X = 9

Sale 4:

Sold for Blizzardy Storm Helmet Without a Home

http://www.tf2outpost.com/trade/29559662 - No B/O

Seller: https://backpack.tf/profiles/76561198050785450#!/compare/1519603200/1519862400

Buyer: https://backpack.tf/profiles/76561198085169466#!/compare/1519516800/1519776000

Best not to use this sale without a B/O, plenty of pure sales to work with here anyways.

Sellers:

~9.9 keys:

https://i.imgur.com/WyaZx7k.png

https://backpack.tf/item/2408943870

11 keys:

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

https://backpack.tf/item/805804649

Sales at:

9

10 (2x)

Going with 9-10.