Credit to suRz for pointing out other sales

History 1:

https://backpack.tf/item/1237642862

16 keys pure:

https://backpack.tf/profiles/76561198002757046#!/compare/1591660800/1591747200

https://backpack.tf/profiles/76561198021055079#!/compare/1591660800/1591747200

22 keys pure:

https://backpack.tf/profiles/76561198021055079#!/compare/1596240000/1596326400

https://backpack.tf/profiles/76561198099685235#!/compare/1596240000/1596326400

Item was then dumped in a Qucksell bot within a day of purchase:

https://backpack.tf/profiles/76561198099685235#!/compare/1596326400/1596412800

https://backpack.tf/profiles/76561198337734785#!/compare/1596326400/1596412800

15 keys pure:

https://backpack.tf/profiles/76561198337734785#!/compare/1598054400/1598140800

https://backpack.tf/profiles/76561198362867182#!/compare/1598054400/1598140800

History 2:

https://backpack.tf/item/1202331638

Item was deposited to Marketplace.tf but unsuccessful, it was then dumped in a Quicksell bot

18 keys pure:

https://backpack.tf/profiles/76561198337734785#!/compare/1596067200/1596153600

https://backpack.tf/profiles/76561198192867202#!/compare/1596067200/1596153600

Dumped back in the same Quicksell bot a week after for 13 keys 11.33 ref, ~13 keys

https://backpack.tf/profiles/76561198192867202#!/compare/1596326400/1596412800

https://backpack.tf/profiles/76561198337734785#!/compare/1596326400/1596412800

Only 3 ref matches in the seller's inventory, the keys were probably instantly traded. Nothing appears in the buyer's inventory either, calling this an untrace

https://backpack.tf/profiles/76561198337734785#!/compare/1596931200/1597017600

https://backpack.tf/profiles/76561198113083770#!/compare/1570060800/1597017600

I'm excluding the sales at 18 keys and 22 keys because they were dumped to Quicksell bots within less than a week of being bought. 13 appears to be a quicksell too

Going with 15-16 keys