Using last month.

Sale 1:

Sold to a bot for 11 keys

Seller: https://backpack.tf/profiles/76561198258264645#!/compare/1524009600/1524096000

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1524009600/1524096000

Bot.tf confirms: https://i.imgur.com/FP8jTHS.png

The seller’s end shows 7 keys, because they bought the Screaming Tiger Runner’s Rhythm from the same bot on the same day. The extra 4 keys can be seen if you paste the Tiger Rhythm’s new ID into the Buyer’s “Near ID filter:

https://backpack.tf/item/6757878524

6788188223

X = 11

Sale 2:

Sold to a bot for 11 keys

Seller: https://backpack.tf/profiles/76561198116110446#!/compare/1523577600/1523664000

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1523577600/1523664000

Untraceable, but bot.tf confirms: https://i.imgur.com/FP8jTHS.png

X = 11

Sale 3:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1524787200/1524873600

Buyer: https://backpack.tf/profiles/76561198080008924#!/compare/1524787200/1524873600

Bot.tf confirms: https://i.imgur.com/FP8jTHS.png

X = 10

Sale 4:

Bulk for stuff

Seller: https://backpack.tf/profiles/76561198220198628#!/compare/1525046400/1525132800

Buyer: https://backpack.tf/profiles/76561198287594985#!/compare/1524700800/1525132800

Sale 5:

Sold for Mega Strike Runner’s Rhythm

Seller: https://backpack.tf/profiles/76561198046845295#!/compare/1524355200/1524614400

Buyer: https://backpack.tf/profiles/76561198072999396#!/compare/1524355200/1524441600

Can be used on the Strike Rhythm.

Sellers:

~9.55 keys:

https://i.imgur.com/6Oi6AZU.png

https://backpack.tf/item/6532227771

Sales at:

10

11 (2x)

Similar situation to this suggestion:

https://backpack.tf/suggestion/5af2a1d0cf6c756a2658a76b

Bot purchases are higher than bot sales, so it should be fine to use them here.

Not using 9.5 as the low-end, as the seller is kinda young, and the value of this unusual according to sales is higher than 10 keys.

Going with 10-11.