Sale 1:

Sold for 10 keys

http://www.tf2outpost.com/trade/27924223 - B/O of 10

Seller: http://backpack.tf/profiles/76561198031007256/#!/compare/1457337600/1457424000

Buyer: http://backpack.tf/profiles/76561198039731024/#!/compare/1457337600/1457424000

X = 10

Sale 2:

Sold for 8 keys + 1 key in items

No outpost link available

Seller: http://backpack.tf/profiles/76561198122156808/#!/compare/1459036800/1459209600

Buyer: http://backpack.tf/profiles/76561198074087200/#!/compare/1458950400/1459036800

X = 8 + 1

X = 9

Sale 3:

Sold for Orbiting Planets Sober Stuntsman + Genuine AWPer Hand

http://www.tf2outpost.com/trade/28000843 - B/O of 11

Seller: http://backpack.tf/profiles/76561198074087200/#!/compare/1459036800/1459123200

Buyer: http://backpack.tf/profiles/76561198198583623/#!/compare/1459123200/1459209600

X = 11 (Capped by B/O)

Sellers:

10 keys:

http://www.tf2outpost.com/trade/28039394 – young, was offered 9 pure a minute after trade was created

12 keys:

http://puu.sh/o6PiH/39b9068d2b.png - young

12.5 keys:

http://puu.sh/o6Pjf/4ff80ce17c.png - 2 days, unbumped

Offers:

http://www.tf2outpost.com/trade/28039017 - 1 day

Sales:

9

10

11

Excluding 9 from the range due to the offer of 9 keys at the 10 key seller

Going with 10-11