Looking at this item: https://backpack.tf/item/1879634736

Sale #1

seller: https://backpack.tf/profiles/76561198045447325#!/compare/1588982400/1589068800

buyer: https://backpack.tf/profiles/76561198055622215#!/compare/1588982400/1589068800

Sold for 13 keys.

Sale #2

seller: https://backpack.tf/profiles/76561198055622215#!/compare/1589241600/1589328000

buyer: https://backpack.tf/profiles/76561198129355329#!/compare/1589241600/1589328000

bulk

Sale #3

seller: https://backpack.tf/profiles/76561198129355329#!/compare/1591142400/1591228800

buyer: https://backpack.tf/profiles/76561198396499936#!/compare/1591142400/1591228800

bulk

Sale #4

seller: https://backpack.tf/profiles/76561198396499936#!/compare/1592956800/1593043200

buyer: https://backpack.tf/profiles/76561198083981694#!/compare/1592956800/1593043200

sold for Circling Heart Cyborg Stunt Helmet + Strange Australium Blutsauger

bulk

Sale #5

seller: https://backpack.tf/profiles/76561198083981694#!/compare/1593043200/1593302400

buyer: https://backpack.tf/profiles/76561198101062081#!/compare/1593129600/1593216000

untrace

Sale #6

seller: https://backpack.tf/profiles/76561198101062081#!/compare/1593216000/1593648000

buyer: https://backpack.tf/profiles/76561198060789752#!/compare/1593561600/1593648000

bulk

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Mp sales: https://marketplace.tf/items/tf2/107;5;u32

Jun 8, 18.00$/1.98$ = 9 keys

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Going with 9-13 keys.