Sale 1:

https://backpack.tf/profiles/76561198316071886#!/compare/1545696000/1545782400

https://backpack.tf/profiles/76561197963396109#!/compare/1545782400/1545868800

Sold for 28 keys

Sale 2:

https://backpack.tf/profiles/76561197963396109#!/compare/1550707200/1550793600

https://backpack.tf/profiles/76561198026744290#!/compare/1550707200/1550793600

Looks like 25? nothing matches.

Sale 3:

https://backpack.tf/profiles/76561198026744290#!/compare/1550880000/1550966400

https://backpack.tf/profiles/76561198011321852#!/compare/1550880000/1550966400

Bulk

Sale 4:

https://backpack.tf/profiles/76561198050518375#!/compare/1545264000/1545350400

https://backpack.tf/profiles/76561198192602363#!/compare/1545264000/1545350400

Sold for ~23 keys

Sale 5:

https://backpack.tf/profiles/76561198192602363#!/compare/1545350400/1545436800

https://backpack.tf/profiles/76561198093518222#!/compare/1545350400/1545436800

Sold for 26 keys

Sale 6:

https://backpack.tf/profiles/76561198093518222#!/compare/1546646400/1546732800

https://backpack.tf/profiles/76561198192602363#!/compare/1546560000/1546646400

QS?

Sale 7:

https://backpack.tf/profiles/76561198192602363#!/compare/1546646400/1546732800

https://backpack.tf/profiles/76561198326910218#!/compare/1546646400/1546732800

Sold for Nether Trail Sombrero

Use there

Sale 8:

https://backpack.tf/profiles/76561198057541872#!/compare/1544832000/1544918400

https://backpack.tf/profiles/76561198072355915#!/compare/1544832000/1544918400

Sold with a key for Phantasm Tin Gallon

Use there

Sale 9:

https://backpack.tf/profiles/76561198072355915#!/compare/1545436800/1545523200

https://backpack.tf/profiles/76561198136290825#!/compare/1545350400/1545436800

Probably 23

Sale 10:

https://backpack.tf/profiles/76561198136290825#!/compare/1545782400/1546300800

https://backpack.tf/profiles/76561197971485686#!/compare/1545955200/1546041600

Sold for 24?

Sale 11:

https://backpack.tf/profiles/76561197971485686#!/compare/1546819200/1546905600

https://backpack.tf/profiles/76561198032176942#!/compare/1546819200/1546905600

Bulk?

Sale 12, 13:

https://marketplace.tf/items/605;5;u32

$45.00 / $1.8 = 25.00 keys

$46.50 / $1.82 = 25.55 keys

Total sales @:

23, 23, 24, 25, 25, 25.55, 26, 28 keys

Unsold for 27 for 2 months,

Buyers up to ~22.5 keys,

Go with a tight range of 24 - 26 keys.