Taking last 2 months.

SALE 1: Unknown

https://backpack.tf/profiles/76561198208312857#!/compare/1564358400/1564444800

https://backpack.tf/profiles/76561198287205590#!/compare/1562457600/1564444800

SALE 2: Nothing seems to match here

https://backpack.tf/profiles/76561198335791307#!/compare/1566691200/1566777600

https://backpack.tf/profiles/76561198407367604#!/compare/1566691200/1566777600

SALE 3: Sold with vivid polar bear for burning polar bear

https://backpack.tf/profiles/76561198194644849#!/compare/1569628800/1569801600

https://backpack.tf/profiles/76561198160856661#!/compare/1569542400/1569628800

Better used for the burning polar bear.

SALE 4: Sold for hot spirit of halloween medi gun(well-worn)

https://backpack.tf/profiles/76561198160856661#!/compare/1570060800/1570147200

https://backpack.tf/profiles/76561198259488531#!/compare/1570060800/1570147200

Medigun is indate at 28 keys.

https://backpack.tf/suggestion/5d7969f7d3817248080f93b6

Buyout was 20 keys

https://bot.tf/stats/listings/4JG2ZLN7

SALE 5: Sold for dbd brain interface

https://backpack.tf/profiles/76561198259488531#!/compare/1570147200/1570233600

https://backpack.tf/profiles/76561198203307876#!/compare/1570060800/1570233600

Interface is unpriced, better used there as fewer sales.

SALE 6: Sold for 17 keys

https://backpack.tf/profiles/76561198203307876#!/compare/1570060800/1570233600

https://backpack.tf/profiles/76561198044684270#!/compare/1570147200/1570233600

SALE 7: Sold for 17 keys

https://backpack.tf/profiles/76561198259790508#!/compare/1566000000/1566518400

https://backpack.tf/profiles/76561198414869460#!/compare/1566518400/1566604800

SALE 8: Sold for strange festivized australium knife

https://backpack.tf/profiles/76561198414869460#!/compare/1566604800/1566691200

https://backpack.tf/profiles/76561198095407289#!/compare/1566604800/1566691200

19.10 rounding to 19 keys.

Festivized australium knife has scm at 19.10 keys, while there's a bot selling for 17.1

https://gyazo.com/83104277c2823caff1fb4995a0eb413f

Taking 19.1 as the seller is extremely young.

SALE 9: Sold with sweets less than 20 refined for 18 keys

https://backpack.tf/profiles/76561198095407289#!/compare/1566604800/1566691200

https://backpack.tf/profiles/76561198349986297#!/compare/1566604800/1566691200

Would round to 18 keys

SALE 10: Sold for spelled items and 4.5 keys in other items

https://backpack.tf/profiles/76561198349986297#!/compare/1566864000/1566950400

https://backpack.tf/profiles/76561198423249928#!/compare/1566864000/1566950400

SALE 11: Sold for 17 keys + 2 refined

https://backpack.tf/profiles/76561198097271466#!/compare/1569283200/1569456000

https://backpack.tf/profiles/76561198423249928#!/compare/1569196800/1569456000

Rounds to 17 keys.

SALE 12: Likely keys, but not sure.

https://backpack.tf/profiles/76561198337734785#!/compare/1565827200/1565913600

https://backpack.tf/profiles/76561198071459079#!/compare/1564704000/1565913600

SALE 13: Unknown

https://backpack.tf/profiles/76561198307920312#!/compare/1568246400/1568332800

https://backpack.tf/profiles/76561198342161127#!/compare/1568160000/1568246400

SALE 14: Nothing seems to match.

https://backpack.tf/profiles/76561198115952983#!/compare/1567296000/1567382400

https://backpack.tf/profiles/76561198306319542#!/compare/1567036800/1567382400

SALE 15: Nothing seems to match

https://backpack.tf/profiles/76561198097271466#!/compare/1566604800/1566691200

https://backpack.tf/profiles/76561197961288175#!/compare/1566604800/1566691200

SALE 16: Sold with 7 keys for scorching scout shako

https://backpack.tf/profiles/76561198054033879#!/compare/1567036800/1567123200

https://backpack.tf/profiles/76561198002757046#!/compare/1567036800/1567123200

Shako is indate at 47 keys.

47 - 7 = 40 keys

No buyout found.

SALE 17: Don't know

https://backpack.tf/profiles/76561198002757046#!/compare/1568246400/1568332800

https://backpack.tf/profiles/76561198110988031#!/compare/1568246400/1568332800

Buyout was 19 keys

https://bot.tf/stats/listings/4KR6QLPN

SALE 18: Sold with taunt: victory lap for something..

https://backpack.tf/profiles/76561198110988031#!/compare/1569628800/1569715200

https://backpack.tf/profiles/76561198349986297#!/compare/1569542400/1569628800

SALE 19: Sold for a max head

https://backpack.tf/profiles/76561198349986297#!/compare/1569715200/1569801600

https://backpack.tf/profiles/76561198174350801#!/compare/1569715200/1569888000

21.3 rounding to 21 keys

No buyout found.

CLASSIFIEDS

https://gyazo.com/b2f8f0afe45457f43545cb590add2d5f

CONCLUSION:

Sales at 17, 17, 17, 18, 19, 20, 21, and 40. Two of the sales at 17 sold relatively quick, however, it seems common enough to include. There is a seller at 20 for 10 days, but 21 seems close enough. 40 is quite high as there are sellers at 20, 25, and 27.

Taking 17-21 keys.