Sale 1:

Sold (within a day of acquiring it) for 9 keys

No outpost link

Seller: http://backpack.tf/profiles/76561198258578200/#!/compare/1460160000/1460246400

Buyer: http://backpack.tf/profiles/76561198165597764/#!/compare/1459900800/1460246400

X = 9

Sale 2:

Sold for 10 keys

http://www.tf2outpost.com/trade/27972077 – B/O of 10, Relisted from http://www.tf2outpost.com/trade/27944847

Seller: http://backpack.tf/profiles/76561198076170767/#!/compare/1458284400/1458370800

Buyer: http://backpack.tf/profiles/76561198171056451/#!/compare/1458284400/1458370800

X = 10

Sale 3:

Sold for 11 keys

No outpost link

Seller: http://backpack.tf/profiles/76561198125791160/#!/compare/1457078400/1457164800

Buyer: http://backpack.tf/profiles/76561198205572052/#!/compare/1457164800/1457251200

X = 11

Sale 4: – Credits to Mr. Pingu

Sold for 12 keys

http://www.tf2outpost.com/trade/28072625 - B/O of 11

Seller: http://backpack.tf/profiles/76561198025486280/#!/compare/1461283200/1461369600

Buyer: http://backpack.tf/profiles/76561198200955590/#!/compare/1460851200/1461456000

http://puu.sh/ouFz6/ec2983474a.jpg

http://puu.sh/ouFA1/3277f26bb4.jpg

Ignoring B/O, pure sale

X = 12

Sale 5:

Sold for Dead Presidents Plug-in Prospector

http://www.tf2outpost.com/trade/27931409 – B/O of 11

Seller: http://backpack.tf/profiles/76561198150783829/#!/compare/1457769600/1457856000

Buyer: http://backpack.tf/profiles/76561198175843135/#!/compare/1457769600/1458025200

X = 11 (Capped by B/O)

Sale 6:

Sold with Strange Killstreak Knife for Blizzardy Storm Hot Dogger

Seller: http://backpack.tf/profiles/76561198276236424/#!/compare/1460073600/1460160000

Buyer: http://backpack.tf/profiles/76561198258578200/#!/compare/1460073600/1460160000

X + 2 = 13

X = 11

Sellers:

12 keys:

http://puu.sh/ouFdx/32a65598b7.png - 2 months

Sales at:

9

10

11

11

11

12

Sale at 9 keys happened very quickly, even without an outpost trade. Excluding 9 keys

Seller at 12 keys for 2 months, excluding 12 keys

Going with 10-11