That seems like an exaggeration. Going over his entire trade history, he's only made a handful of trades for established NHLers.
Pearson for Gudbranson = win
Spooner for Gagner = wash
Schenn for Del Zotto = win
Leivo for Carcone = win
Leipsic for Holm = win (albeit a very minor one)
Gudbranson & 5th for McCann & 2nd & 4th = loss
Sutter & 3rd for Bonino, Clendening & 2nd = loss
Prust for Kassian = loss
Bonino, 1st, Sbisa and 3rd for Kesler & 3rd = loss (Though Benning was obviously extremely handcuffed here)
Dorsett for 3rd = win
As far as I can tell, these are all of the trades Benning has made for somewhat established players.
5 wins, 1 tie, 4 losses.
So the argument that Benning has lost every trade for an established player isn't a very strong one. If you want to make the argument that Benning hasn't won any of the bigger trades he's made, I think there's a stronger argument to be made.