Using sales from April 20th and so on.

Sale 1: Untrace.

https://backpack.tf/profiles/76561198067002150#!/compare/1555891200/1555977600

https://backpack.tf/profiles/76561198032176942#!/compare/1555891200/1555977600

Sale 2: ~14 Keys Pure.

https://backpack.tf/profiles/76561198341171690#!/compare/1556582400/1556668800

https://backpack.tf/profiles/76561198105220551#!/compare/1556150400/1557705600

https://bot.tf/stats/item?def=31000&q=5&ef=Head+of+Steam&craft=1&aus=0&ks=0

Sale 3: ~14 Keys Pure.

https://backpack.tf/profiles/76561198928347702#!/compare/1558569600/1558656000 -- ID --> 7793914615

https://backpack.tf/profiles/76561198869144080#!/compare/1558656000/1558742400

Sale 4: Bulk/Use There.

https://backpack.tf/profiles/76561198869144080#!/compare/1559779200/1559865600

https://backpack.tf/profiles/76561198280357510#!/compare/1559779200/1559865600

Sale 5: ~12 Keys.

Sold For Strange Specialized Killstreak Australium Grenade Launcher (14)

B/O Cap 12, https://bot.tf/stats/listings/4Z20QD1M

https://backpack.tf/profiles/76561198280357510#!/compare/1560038400/1560124800

https://backpack.tf/profiles/76561198242866846#!/compare/1559606400/1560124800

Sale 6: Bulk.

https://backpack.tf/profiles/76561198242866846#!/compare/1560211200/1560297600

https://backpack.tf/profiles/76561198107839402#!/compare/1560124800/1560211200

Sale 7: Bulk.

https://backpack.tf/profiles/76561198107839402#!/compare/1560902400/1560988800

https://backpack.tf/profiles/76561198128876846#!/compare/1560902400/1560988800

Sale 8: Untrace/QS.

https://backpack.tf/profiles/76561198064255155#!/compare/1557273600/1557360000

https://backpack.tf/profiles/76561198312583107#!/compare/1557187200/1557360000

Sale 9: Bulk/Untrace.

https://backpack.tf/profiles/76561198315335737#!/compare/1556668800/1559001600

https://backpack.tf/profiles/76561198154700747#!/compare/1558915200/1559001600

Sale 10: Bulk. (Stopping eating my brain)

https://backpack.tf/profiles/76561198154700747#!/compare/1559260800/1559347200

https://backpack.tf/profiles/76561198114245825#!/compare/1559260800/1559347200

Sale 11: ~12 Keys Pure.

https://backpack.tf/profiles/76561198318531659#!/compare/1557100800/1557187200

https://backpack.tf/profiles/76561198168425477#!/compare/1557100800/1557187200

Sale 12:

Sold For Blizzardy Storm Otolaryngologist's Mirror. (Seems low)

https://backpack.tf/profiles/76561198168425477#!/compare/1557187200/1557532800

https://backpack.tf/profiles/76561198236788328#!/compare/1557446400/1557532800

https://marketplace.tf/items/31000;5;u113

Sale 13: ~16 Keys. (High)

Apr 25

$29.30 / $1.79 = 16.37 keys

Sale 14: ~15 Keys. (Calling high as well, 2 months old sellers at 15 keys)

Jun 2

$28.00 / $1.81 = 15.47 keys.

Sales: 12/12/14/14/15/16

Going with 12-14 keys.

Thanks to scourge for the premium!