Sale 1:

https://backpack.tf/profiles/76561198236985112#!/compare/1579651200/1580256000

6 keys and 18 refs, which is 6.33 keys = 6.5 keys

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Sale 2:

https://backpack.tf/profiles/76561198878948902#!/compare/1583020800/1583107200

https://backpack.tf/profiles/76561198236985112#!/compare/1580256000/1583107200

unknown

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Sale 3:

https://backpack.tf/profiles/76561198878948902#!/compare/1583107200/1583193600

https://backpack.tf/profiles/76561198850274253#!/compare/1583107200/1583193600

sold to a quickbuyer bot with other items

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Sale 4:

https://backpack.tf/profiles/76561198061353482#!/compare/1585440000/1586304000

https://backpack.tf/profiles/76561198850274253#!/compare/1586217600/1586304000

no idea

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Sale 5:

https://backpack.tf/profiles/76561198061353482#!/compare/1586304000/1586476800

https://backpack.tf/profiles/76561198986747837#!/compare/1586131200/1586304000

idk

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Sale 6:

https://backpack.tf/profiles/76561198986747837#!/compare/1586304000/1586476800

https://backpack.tf/profiles/76561198030137422#!/compare/1586217600/1586304000

idk

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Sale 7:

https://backpack.tf/profiles/76561198030137422#!/compare/1586304000/1586390400

https://backpack.tf/profiles/76561198382034007#!/compare/1586217600/1586304000

moved to their bot

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Sale 8:

https://backpack.tf/profiles/76561198048851745#!/compare/1586304000/1586476800

https://backpack.tf/profiles/76561198030137422#!/compare/1586304000/1586390400

idk

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Sale 9:

https://backpack.tf/profiles/76561197999507606#!/compare/1578614400/1582761600

no clue, bulk

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Sale 10:

https://backpack.tf/profiles/76561198970987097#!/compare/1582588800/1583280000

https://backpack.tf/profiles/76561197999507606#!/compare/1582761600/1585267200

mistery

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Sale 11:

https://backpack.tf/profiles/76561198970987097#!/compare/1585094400/1585267200

https://backpack.tf/profiles/76561198177719569#!/compare/1585008000/1585094400

some bulk deal

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Sale 12:

https://backpack.tf/profiles/76561198348889959#!/compare/1584230400/1584316800

https://backpack.tf/profiles/76561198311498888#!/compare/1584144000/1584230400

i just dunno

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Sale 13:

https://backpack.tf/profiles/76561198348889959#!/compare/1584316800/1584489600

https://backpack.tf/profiles/76561198453530349#!/compare/1584316800/1584403200

didn't find anything, but maybe i'm just blind

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Sale 14:

https://backpack.tf/profiles/76561199017926833#!/compare/1585353600/1585526400

https://backpack.tf/profiles/76561198453530349#!/compare/1585267200/1585440000

shrug

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Sale 15:

https://backpack.tf/profiles/76561199017926833#!/compare/1585526400/1585699200

https://backpack.tf/profiles/76561198052324591#!/compare/1577836800/1585612800

doesn't even show up

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Sale 16:

https://backpack.tf/profiles/76561198878948902#!/compare/1585440000/1585612800

idk

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Sale 17:

https://backpack.tf/profiles/76561198036144976#!/compare/1585958400/1586131200

https://backpack.tf/profiles/76561198061694931#!/compare/1585958400/1586131200

same

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Sale 18:

https://backpack.tf/profiles/76561198986747837#!/compare/1586304000/1586476800

https://backpack.tf/profiles/76561198061694931#!/compare/1586304000/1586476800

smh.. idk

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Sale 19:

https://backpack.tf/profiles/76561198141477086#!/compare/1586304000/1586476800

https://backpack.tf/profiles/76561198986747837#!/compare/1586390400/1586476800

u tell me

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Sale 20:

https://backpack.tf/profiles/76561198305494036#!/compare/1586649600/1586822400

https://backpack.tf/profiles/76561198976233202#!/compare/1586649600/1586736000

c l u e l e s s

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Sale 21:

https://backpack.tf/profiles/76561198317165564#!/compare/1586736000/1586822400

bulk

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Sale 22:

https://backpack.tf/profiles/76561198089253101#!/compare/1579651200/1579824000

https://backpack.tf/profiles/76561198037674114#!/compare/1579305600/1580342400

bulk

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Sale 23:

https://backpack.tf/profiles/76561198037674114#!/compare/1583107200/1583884800

https://backpack.tf/profiles/76561198337734785#!/compare/1583798400/1583971200

doesn't even show up

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Sale 24:

https://backpack.tf/profiles/76561198337734785#!/compare/1586476800/1586563200

idk bots doing bot things

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Sale 25:

https://backpack.tf/profiles/76561198080179568#!/compare/1586476800/1586563200

There's 2x 6 keys and 16 refs from Killua, so one of them prolly (6.33k=6.5k), worth mentioning that his buyorder is at that so gotta be correct

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Sale 26:

https://backpack.tf/profiles/76561198202133459#!/compare/1581206400/1581897600

7 keys

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Sale 27:

https://marketplace.tf/items/tf2/30912;5;u6

13$ on Jan 26

https://marketplace.tf/items/tf2/5021;6

13/1.78=7.3 keys=7.5 keys

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Sale 28:

https://marketplace.tf/items/tf2/30912;5;u6

12.71$ on Apr 6

https://marketplace.tf/items/tf2/5021;6

12.71/1.69=7.5 keys

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Sale 29:

https://marketplace.tf/items/tf2/30912;5;u6

12.71$ on Apr 8

https://marketplace.tf/items/tf2/5021;6

12.71/1.69=7.5 keys

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I spent too much time doing this, couldn't really find normal sales, however since there is an unsuccessful seller at 6.26, which is closer to 6.5 then 6 keys, and we do have actually two sales around 6.5, I'd go with 6.5 flat.

Mods, help :/