Sale 1:

Sold for Terror-Watt Sober Stuntsman + 1 key

No outpost

Seller: http://backpack.tf/profiles/76561198136954791#!/compare/1465862400/1466899200

Buyer: http://backpack.tf/profiles/76561198138724799#!/compare/1466812800/1466899200

X = 16

Sale 2:

Sold for 14 keys

http://www.tf2outpost.com/trade/28226731 - B/O of 14

Seller: http://backpack.tf/profiles/76561197993826799#!/compare/1465257600/1465344000

Buyer: http://backpack.tf/profiles/76561198210692339#!/compare/1465171200/1465344000

X = 14

Sale 3:

Sold for Terror-Watt Cyborg Stunt Helm + 1 key in items

http://www.tf2outpost.com/trade/28287713 - B/O of 14

Seller: http://backpack.tf/profiles/76561198138724799#!/compare/1466812800/1466899200

Buyer: http://backpack.tf/profiles/76561198109727823#!/compare/1466899200/1466985600

X = 13

Sale 4:

Sold with Killstreak Current Event for Bubbling El Jefe

No outpost

Seller: http://backpack.tf/profiles/76561198028274974#!/compare/1463184000/1463270400

Buyer: http://backpack.tf/profiles/76561198079098551#!/compare/1463184000/1463356800

X + 4 = 16

X = 12

Sellers:

16 keys:

http://puu.sh/pLppU/31f6949588.png

17 keys:

http://puu.sh/pLpoX/c3dcb0133e.png - 1 month

Buyer at 12 keys:

http://www.tf2outpost.com/trade/27817199

Sales at:

12

13

14

16

Buyer at 12, excluding 12

Going with 13-16