Sale 1:

Sold for Aces High Festive Tree (19) + 2 keys + 10 ref (<0.5)

Not gonna bother searching outpost, user never displays B/O’s in trades.

Seller: http://backpack.tf/profiles/76561198011321852#!/compare/1481241600/1481328000

Buyer: http://backpack.tf/profiles/76561198287942889#!/compare/1480896000/1481414400

X = 21

Sale 2:

Sold for Orbiting Fire Buccaneer’s Bicorne (18.5) + <2 keys in items

No outpost

Seller: http://backpack.tf/profiles/76561198200090441#!/compare/1484611200/1484784000

Buyer: http://backpack.tf/profiles/76561198184105439#!/compare/1484784000/1484870400

X = 20

Sale 3:

Sold for 20 keys

http://www.tf2outpost.com/trade/28588139 - B/O of 20

Seller: http://backpack.tf/profiles/76561198137755376#!/compare/1484265600/1484524800

Buyer: http://backpack.tf/profiles/76561198190450275#!/compare/1484524800/1485820800

X = 20

Sale 4:

Sold for Cloud 9 Glasgow Great Helm

Seller: http://backpack.tf/profiles/76561198061464918#!/compare/1485561600/1485648000

Buyer: http://backpack.tf/profiles/76561198195483499#!/compare/1485475200/1485648000

Can be used on the Glasgow

Sellers:

18 keys:

http://i.imgur.com/YqEaAtC.png

http://backpack.tf/u/76561198195483499

http://www.tf2outpost.com/trade/28748659 - 2 days

http://i.imgur.com/Xi9anRn.png

http://backpack.tf/u/76561198184105439

http://www.tf2outpost.com/trade/28749200 - 2 days, lowered from 20 keys here: http://www.tf2outpost.com/trade/28735190

Sales at:

20 (2x)

21

Sellers are too young to be included in the range.

Going with 20-21.