Why not

Sale #1: 5 buds

http://www.tf2outpost.com/trade/23122233

Sold for a Sunbeams Chieftain's Challenge

http://i.imgur.com/HeZokp0.png

6 * 0.9 = 5.4 buds

B/O of 5 buds

Sale #2: 3 buds (outlier)

http://www.tf2outpost.com/trade/23122233 (no outpost trade for the person who sold it to him)

Sold for a Level 1 Max’s Severed Head

Backpack Before: http://backpack.tf/profiles/76561198111411801?time=1412665200

Backpack After: http://backpack.tf/profiles/76561198111411801?time=1413270000

3.3 * 0.9 = 2.97 rounding to 3 buds

Sale #3: 5.3 buds

http://www.tf2outpost.com/trade/22920601

Sold for Smoking Chieftain's Challenge, Genuine Horace, and a Clean Max’s Severed Head

Backpack Before: http://backpack.tf/profiles/76561197990172296?time=1411974000

Backpack After: http://backpack.tf/profiles/76561197990172296?time=1412060400

2.2 * 0.9 = 1.98 buds rounding to 2 buds

2 + 3.3 = 5.3 buds