Well Dc lucks out in trades because you can find a variety of them in stores unlike Marvel where you find Avengers, Spider-Man, Fantastic Four, X-Men, Captain America & Black Panther.
Of the new 52-I can say I have seen at least 1 trade of every series except-Green Lantern Corps, Supergirl, Team 7 (is one out yet?, World's Finest & Earth 2.
Of the old DC Universe I think the only trades I did not see was Wonder Woman's last series, the final run of Teen Titans & pretty much all the final trades (except Xombi) before the new 52.
Of Marvel-She Hulk, Young Avengers, Blade, Defenders, Dr. Strange, Silver Surfer, Guardians of the Galaxy, Nova, X-Force (the early years), the Bendis era on Moon Knight, Captain Marvel, Red Hulk, Journey in to Mystery, X-Factor (Peter David era including his 90s run), Avengers Arena (I think one of those volumes is out) & Fantastic Foundation are MIA. Miles Morales's trades-LOL is the hardest trade to find in stores.
Heck IDW, Kaboom Boom & Archie trades are easier to find than some Marvel trades.