Another sale occurred after I posted my other suggestion for this.

Sale #1:

http://www.tf2outpost.com/trade/26102726

Sold for 13 keys

Seller:

http://backpack.tf/profiles/76561198042938501?time=1433314800 (June 3rd)

http://backpack.tf/profiles/76561198042938501?time=1433401200 (June 4th)

Buyer:

http://backpack.tf/profiles/76561198142812247?time=1433142000 (June 1st)

http://backpack.tf/profiles/76561198142812247?time=1433401200 (June 4th)

Steaming Birdcage and other items seem to have sold on the same day to make 13 keys. 13 keys from the buyer can be tracked from the seller's inventory.

Sale #2:

http://www.tf2outpost.com/trade/25885803

Sold for 13 keys

http://i.imgur.com/1xuJOFE.png

Sale #3:

http://www.tf2outpost.com/trade/26197514

Sold for 10 keys + ~5 keys in items

http://backpack.tf/profiles/76561198052682328?time=1434265200 (June 14th)

http://backpack.tf/profiles/76561198052682328?time=1434351600 (June 15th)

Items received: http://i.imgur.com/AbWikqp.png

Closer to 15 keys