Thanks to Mr. Vortex and d0 for the help.

http://www.tf2outpost.com/trade/22778456

sold for aussie scatter

seller after: http://backpack.tf/profiles/76561198050753967?time=1411714800

seller before: http://backpack.tf/profiles/76561198050753967?time=1411542000

price: 2.7 buds

http://www.tf2outpost.com/trade/22583948

3 buds 5 keys http://snag.gy/OfQIp.jpg

seller after: http://backpack.tf/profiles/76561198059714651?time=1411282800

seller before: http://backpack.tf/profiles/76561198059714651?time=1411101015

Price: 3.3 buds

http://www.tf2outpost.com/trade/23285139

sold with 2 buds for a planets stash. He offered; no need for calculating overpay

Price: 5-2 = 3 buds

http://www.tf2outpost.com/trade/22806284

had a b/o of 4 buds; sold for kaw ninja hood.

Price after overpay : 0.9*5.5 = 4.5

Price: 4 buds (from B/O)

Why 2.6 is an outlier and not 4:

1. aussie scatter sale happened in 2 days

2. there is a 3 pure offer for the 4th trade, implying anything below 3 can be ignored, and 3 can be considered the lower range.