Sale 1:

Sold for 28 keys

http://www.tf2outpost.com/trade/29548447 - B/O of 28

Seller: https://backpack.tf/profiles/76561198071841386#!/compare/1519084800/1519171200

Buyer: https://backpack.tf/profiles/76561197991916377#!/compare/1519084800/1519171200

X = 28

Sale 2:

Sold for 26 keys

Seller: https://backpack.tf/profiles/76561198115721685#!/compare/1517788800/1517875200

Buyer: https://backpack.tf/profiles/76561198071841386#!/compare/1517702400/1517875200

X = 26

Sale 3:

Sold in 2 days for 21 keys

Seller: https://backpack.tf/profiles/76561198030343929#!/compare/1520812800/1520899200

Buyer: https://backpack.tf/profiles/76561198168625615#!/compare/1520812800/1520899200

X = 21

Sale 4:

Quicksold for 21 keys (Buyer’s end)

Seller: https://backpack.tf/profiles/76561198030343929#!/compare/1519430400/1519516800

Buyer: https://backpack.tf/profiles/76561198149211008#!/compare/1519430400/1519516800

X = 21

Sale 5:

Sold in ~2 days for 20 keys

Seller: https://backpack.tf/profiles/76561198030343929#!/compare/1521158400/1521244800

Buyer: https://backpack.tf/profiles/76561198255850609#!/compare/1521158400/1521244800

X = 20

Sale 6:

Quicksold for 19 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198039548756#!/compare/1520899200/1521072000

Buyer: https://backpack.tf/profiles/76561198030343929#!/compare/1520985600/1521072000

X = 19

Sale 7:

Sold for Peace Dragonborn + 4 keys + cheap crates

Seller: https://backpack.tf/profiles/76561198255850609#!/compare/1521158400/1521244800

Buyer: https://backpack.tf/profiles/76561198035458322#!/compare/1521158400/1521244800

Supports low-end.

Sale 8:

Bulk for Cloud 9 Balloonihoodie

Seller: https://backpack.tf/profiles/76561198025367258#!/compare/1520380800/1520467200

Buyer: https://backpack.tf/profiles/76561198095902384#!/compare/1520380800/1520467200

Sellers:

23 keys:

http://www.tf2outpost.com/search/273572317

25 keys:

https://i.imgur.com/hipzmTP.png

https://backpack.tf/item/3277250932

https://i.imgur.com/hDW1njp.png

https://backpack.tf/item/3268820678

32 keys:

https://i.imgur.com/s9ubGLZ.png

https://backpack.tf/item/5986206331

Sales at:

19

20

21 (2x)

26

28

Sales at 21 and lower were quicksales, though they outnumber the sales at higher than 21, and there are several sellers for cheaper than the non-quicksales. Wide range here might be necessary. Including the highest quickselling point (21).

Going with 21-28.