Seems right, Two sellers on Outpost with B/O's of 4. Ya Done Good Kid

You sold it for Blizzardy Storm Hound Dog + Blizzardy Tam O' Shanter and a Bubbling Triboniophorus Tyrannus = ~4 pure.

Link 1 (B/O of 3.5) has sold for 2 buds + 26 Keys + a strange huntsman = ~3.3 pure.

http://www.tf2outpost.com/trade/7851118 - b/o of 4 now

http://www.tf2outpost.com/trade/6515767 - b/o of 4 too

All in all I think this is acceptable, even if I would prefer a low value of 3.3 or 3.4 instead of 3.5, but given the fact that his b/o was too low I agree with you. :)