Probably one of the toughest suggestions I've made.

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Sold with 3 keys for Blizzardy Storm Human Cannonball (31):

http://backpack.tf/profiles/76561198094881984?time=1425456000 (March 4th)

http://backpack.tf/profiles/76561198094881984?time=1425542400 (March 5th)

31 - 3 = 28 keys

[Sold with 4 keys in items + 2 Earbuds + 2 keys] for Disco Beat Down Human Cannonball (40):

http://backpack.tf/profiles/76561198137732653?time=1429599600 (April 21st)

http://backpack.tf/profiles/76561198137732653?time=1429686000 (April 22nd)

40 - 4 - 2 - 2 = 32 keys

Sold for Orbiting Fire Flipped Trilby (~25?) + 4 keys in items:

http://www.tf2outpost.com/trade/25868475

http://backpack.tf/profiles/76561198155722861?time=1430722800 (May 4th)

http://backpack.tf/profiles/76561198155722861?time=1430809200 (May 5th)

O Fire Trilby is probably lower and this sale is probably an outlier. However, buyout of 32 keys.

Sold for Strange Specialized Killstreak Australium Scattergun (36):

http://backpack.tf/profiles/76561197998782246?time=1429772400 (April 23rd)

http://backpack.tf/profiles/76561197998782246?time=1430204400 (April 28th)

36 * .9 = 32.4 keys

Rounded to 32 keys

Unsold at 38, then 30 keys:

http://www.tf2outpost.com/trade/25804710

http://www.tf2outpost.com/trade/25802772

http://www.tf2outpost.com/trade/25802232

http://www.tf2outpost.com/trade/25792385

http://www.tf2outpost.com/trade/25782209

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Strange Specialized Killstreak Australium Scattergun:

http://backpack.tf/vote/id/555470bdb98d882a2b8b4600