Sale #1:

http://www.tf2outpost.com/trade/26027136

Sold for Pro KS SF Shotgun (~12?) + TS Paint (1.15) + Bill's (2) + 26 keys

http://backpack.tf/profiles/76561198202280436?time=1433055600 (May 31st)

http://backpack.tf/profiles/76561198202280436?time=1433228400 (June 2nd)

http://www.tf2outpost.com/trade/25934554 Had a B/O of 31 pure here

Exceeds the B/O of 31 keys

Sale #2:

http://www.tf2outpost.com/trade/26115653

Sold with a Max Head (24) for a PE Trophy Belt (57.5)

http://backpack.tf/profiles/76561198096299803?time=1433314800 (June 3rd)

http://backpack.tf/profiles/76561198096299803?time=1433401200 (June 4th)

http://www.tf2outpost.com/trade/26098953 Trophy

(57.5 * 0.9) - 24 = 27.75; rounded to 28 keys

Sale #3:

http://www.tf2outpost.com/trade/26042595

Sold for Steaming Luchadore + Heart Mongolian + ~2.5 keys in other items

http://backpack.tf/profiles/76561198139815791?time=1433228400 (June 2nd)

http://backpack.tf/profiles/76561198139815791?time=1433401200 (June 4th)

Both the Luchadore and Mongolian are heavily outdated, but I could see this being around the range.

http://backpack.tf/vote/id/5571bbaddea9e995648b4586 Another sale around 28 keys in the previous suggestion as well.

Some Sellers:

http://www.tf2outpost.com/trade/26020766 22 keys, seems like this would've sold in the time it's been up

http://www.tf2outpost.com/trade/25952570 31 keys, 3 weeks unsold

http://www.tf2outpost.com/trade/25950554 34 keys, 3 weeks unsold, 0 offers

http://www.tf2outpost.com/trade/26116429 34 keys

http://www.tf2outpost.com/trade/26121582 35 keys or a Max + 5 keys (~29)

http://www.tf2outpost.com/trade/26121017 35 keys, was offered 22 instantly