Challenge 19, Team F.L.A.M.B.E.S!

69th(nice) suggestion, made by Beaser!

https://marketplace.tf/items/tf2/30413;5;u7

$44.75 / $1.79 = 25.00 keys

$45.75 / $1.87 = 24.47 keys

$59.99 / $1.88 = 31.91 keys

$49.95 / $1.98 = 25.23 keys

Unable to obtain histories

https://imgur.com/d3AtBwP - 26 keys

https://backpack.tf/profiles/76561198928421771#!/compare/1588982400/1589068800

https://backpack.tf/profiles/76561198081962061#!/compare/1588550400/1589155200

Sale #1 -

23 keys

https://backpack.tf/profiles/76561198388363768#!/compare/1590537600/1590624000

https://backpack.tf/profiles/76561198021228339#!/compare/1589587200/1590624000

Sale #2 -

QS

https://backpack.tf/profiles/76561198082874812#!/compare/1590278400/1590451200

https://backpack.tf/profiles/76561198388363768#!/compare/1590364800/1590451200

Sale #3 -

Untrace

https://backpack.tf/profiles/76561198154700747#!/compare/1590105600/1590278400

https://backpack.tf/profiles/76561198163544364#!/compare/1588204800/1590624000

Sale 4:

https://backpack.tf/profiles/76561198850274253#!/compare/1585440000/1585526400

https://backpack.tf/profiles/76561198335431391#!/compare/1585440000/1585526400

Sold for ~22 keys

sale 5:

https://backpack.tf/profiles/76561198082874812#!/compare/1590278400/1590451200

https://backpack.tf/profiles/76561198388363768#!/compare/1590278400/1590451200

sold for ~19 keys

Sale 6:

https://backpack.tf/profiles/76561198069044978#!/compare/1592352000/1592697600

https://backpack.tf/profiles/76561198061779536#!/compare/1592611200/1592697600

bulk

Sales at 19, 22, 22, 23, 24, 25, 25, 26, and 32

buyers up to ~18.5 keys,

seller at 35 keys,

Excluding 32, 19 and going with 22 - 26 keys.