Last sugg on this accepted a month ago, but used sales now out of date.

Sale 1-6:

https://marketplace.tf/items/tf2/1111;5;u3026

$15 / $1.78 = 8.43 keys // https://backpack.tf/item/3237700072

$6.11 / $1.73 = 3.53 keys

$7 / $1.69 = 4.14 keys

$10 / $1.68 = 5.95 keys // https://backpack.tf/item/6180955588

$11.25 / $1.69 = 6.66 keys

$11.36 / $1.70 = 6.68 keys

Sale 7:

bulk

https://backpack.tf/profiles/76561198815283793#!/compare/1580947200/1581206400

https://backpack.tf/profiles/76561198311437371#!/compare/1581120000/1581206400

Sale 8:

bulk

https://backpack.tf/profiles/76561198311437371#!/compare/1581897600/1582070400

https://backpack.tf/profiles/76561198012626449#!/compare/1581033600/1582329600

Sale 9:

bulk

https://backpack.tf/profiles/76561198236007746#!/compare/1579305600/1579392000

https://backpack.tf/profiles/76561197980761347#!/compare/1579219200/1579392000

Sale 10:

untrace

https://backpack.tf/profiles/76561198236007746#!/compare/1579392000/1579478400

https://backpack.tf/profiles/76561198818590510#!/compare/1582761600/1582848000

Sale 11:

untrace

https://backpack.tf/profiles/76561198818590510#!/compare/1584576000/1584662400

https://backpack.tf/profiles/76561198005032783#!/compare/1584576000/1584662400

Sale 12:

SCM

https://backpack.tf/profiles/76561198005032783#!/compare/1584662400/1584748800

https://backpack.tf/profiles/76561198055058172#!/compare/1583884800/1584921600

Sale 13:

Nether Void Second Rate Sorcery(5.75)

https://backpack.tf/profiles/76561198279670065#!/compare/1581120000/1581206400

https://backpack.tf/profiles/76561198223239048#!/compare/1580947200/1581206400

Sale 14:

better used on flip

https://backpack.tf/profiles/76561198223239048#!/compare/1580947200/1581206400

https://backpack.tf/profiles/76561198859721638#!/compare/1581206400/1581292800

Sale 15:

bulk

https://backpack.tf/profiles/76561198820768973#!/compare/1581292800/1581379200

https://backpack.tf/profiles/76561198859721638#!/compare/1581292800/1581379200

Sale 16:

untrace

https://backpack.tf/profiles/76561198820768973#!/compare/1581552000/1581638400

https://backpack.tf/profiles/76561198072441047#!/compare/1581552000/1581638400

Sale 17:

bulk

https://backpack.tf/profiles/76561198072441047#!/compare/1581724800/1582156800

https://backpack.tf/profiles/76561198005032783#!/compare/1582070400/1582156800

==============================================================================

Sales:

3.5, 4.1, 6, 6, 6.5, 6.5, 8.5

buyers at 4.2: https://imgur.com/a/YWeOB9J