Resuggesting with a range.

Sale #1:

Offered with 2 keys for an Orbiting Planets Tyrant's Helm:

http://www.tf2outpost.com/trade/26617739

Total: 16 - 2 = 14 keys

Sale #2: (Credits to DedAndrei)

Sold for an Orbiting Fire Ye Olde Baker Boy and negligible sweets:

http://prntscr.com/8pciap

Seller's backpack before: http://backpack.tf/profiles/76561198158396145?time=1444287600

Seller's backpack after: http://backpack.tf/profiles/76561198158396145?time=1444201200

Total 19.5*0.9 = 17.55 keys

Rounding it up to 18 keys

Please tell me if I have made a mistake.