Sale 1: (Dupe)

Sold for Disco Beat Down Liquidator’s Lid (27)

User doesn’t list B/O’s on outpost trades, no point looking for the outpost trade.

Seller: http://backpack.tf/profiles/76561198011321852#!/compare/1488326400/1488412800

Buyer: http://backpack.tf/profiles/76561198124055526#!/compare/1488412800/1488499200

X = 27

Sale 2:

Sold with 5 keys for Haunted Ghost Bonk Helm (27)

http://www.tf2outpost.com/trade/28833454 - B/O of 26

Seller: http://backpack.tf/profiles/76561198118787406#!/compare/1489536000/1489622400

Buyer: http://backpack.tf/profiles/76561197963500354#!/compare/1489536000/1489622400

X = 22

Sale 3:

Some bulk sale

Seller: http://backpack.tf/profiles/76561198047239660#!/compare/1488585600/1488672000

Buyer: http://backpack.tf/profiles/76561198080786736#!/compare/1488672000/1489276800

Sellers:

20.5 keys:

http://i.imgur.com/DNu9riu.png - 4 days

http://backpack.tf/u/76561197963500354

22 keys:

http://i.imgur.com/am41ciz.png - 5 days

http://backpack.tf/u/76561198119563409

24 keys:

http://www.tf2outpost.com/trade/28905911 - 2 days, re-list of

http://www.tf2outpost.com/trade/28815508 - 27 keys for a month

http://backpack.tf/u/76561198080786736

Offers:

http://www.tf2outpost.com/trade/28897021 - Re-listed many times over 2 months, never a B/O listed

http://backpack.tf/u/76561198202391840

Sales at:

22

27

Excluding 27, many sellers for less than 27.

Seller at 20.5, rounding to 21.

Going with 21-22.

Cheers Erik for helping me out with the range!