Other sales -

Sale 1 - 11 keys

https://marketplace.tf/items/30920;5;u3001

Sold for $22.50 ( / $1,98 ) = 11 keys.

Sale 2 - 10 keys

https://backpack.tf/profiles/76561198069844957#!/compare/1524787200/1524873600

https://backpack.tf/profiles/76561198350044920#!/compare/1524787200/1524873600

http://www.tf2outpost.com/trade/29627020 - B/O 20 keys

Sold along an Uncraftable Summer Hat (~6) + Victory Lap (~3) (~9) for a Haunted Ghosts Stately Steel Toe (19) = 10 keys.

Mini for the Haunted Ghosts Stately Steel Toe = 19 keys

https://backpack.tf/profiles/76561198069844957#!/compare/1525046400/1525219200

https://backpack.tf/profiles/76561198114402400#!/compare/1525132800/1525219200

Sold along 12 keys for a Circling Hearts Honcho's Headgear (31) = 19 keys. Current sellers on the Toe are young.

Sale 3 - invalid

https://backpack.tf/profiles/76561198089675316#!/compare/1525392000/1525478400

https://backpack.tf/profiles/76561198069813252#!/compare/1525392000/1525478400

Bulk.

Sale 4 - 17 keys

https://backpack.tf/profiles/76561198305676157#!/compare/1523664000/1523750400

https://gyazo.com/f93360b0e02614f5f279738779667d06

Sold for 17 (dates match) - seems high.

Sale 5 & 6 - 11 + 11 keys

https://backpack.tf/profiles/76561198305676157#!/compare/1525132800/1525219200

https://gyazo.com/f93360b0e02614f5f279738779667d06

Sold twice by the same bot for 11 keys. Dates match with the screenshot.

Sale 7 & 8 & 9 - 10 + 10 + 10 keys

https://backpack.tf/profiles/76561198305676157#!/compare/1523836800/1523923200

https://backpack.tf/profiles/76561198305676157#!/compare/1521590400/1521676800

https://gyazo.com/f93360b0e02614f5f279738779667d06

Bought two for 10 keys each. Also bought one for 10 keys earlier in March.

Sale 11 - invalid

https://backpack.tf/profiles/76561197995973040#!/compare/1521244800/1521331200

Unusualified

Sale 12 - invalid

https://backpack.tf/profiles/76561198174759381#!/compare/1525910400/1526083200

Sold within a day for 8.5 keys. Excluding.

Sale 13 - invalid

https://backpack.tf/profiles/76561198056531196#!/compare/1525824000/1525910400

Unusualified after the person bought it off the bot (you can still it's still the unique version before it sold to the bot).

So the sales here are -

8.5 / 10 / 10 / 10 / 10 / 11 / 11 / 11 / 17

Current seller: 43 keys

https://gyazo.com/1cce579656dde2adbed77f9be7a1b9e0

8.5 sold within a day so I'm inclined to exclude that one. The 17 key sale also seems a bit high here compared to the massive amount of sales below 17 keys. 10-11 keys seems to be the most common trading point.

I would personally go with a 10-11 key range.