I think the New 52 sales are still strong for a core batch of collected editions of New 52 material, but on the flip side
the reboot killed older collected edition sales for sure. I encourage and talk up the classics like the Tales of the Batman collections, Green Lantern/Green Arrow, Kirby's Fourth World, The Legion: Great Darkness Saga, etc, and I still sell them, but only on the praise of the fact they are great stories. That's certainly meritorious reason enough for any book to find a reader, but that leaves many other books languishing on the shelves.
No one knows where they fit or if they happened or matter at all to the New 52, and lacking the spotlight of fan/reader appreciation most folks ignore them. I don't have that problem with older Marvel collected editions. In fact they move very well for us! I think part of what contributes to their sales is the books offer stories that still matter! Old heroes and villains origins are still largely intact, the events of them or the teams they serve on still resonate in the current day's books with aspects like The New Universe characters, Captain Universe, World War II, the original Nick Fury, the creation of Ultron, the Dark Phoenix Saga, etc, etc, etc still echoing through today's Marvel Universe. We all know where these things come in and that they happened and carry weight! With DC Comics you have so many books that read like they have their basis in a character forged after a lightning round of writers half studying the publisher's intellectual properties' histories via their Wiki entries and cutting anything that won't sound slick when characters are added to the next animated feature or video game. Sure you can say, well this did happen but that was before Flashpoint or that was before Infinite Crisis or before Zero Hour, but hasn't that been part of the problem for DC since Crisis on Infinite Earths? Nowadays being a fan of superhero comic books makes me envy sports fans because at least no editorial retreat creates a cosmic event that wipes out the history of the NBA, NFL, or MLB. I think this also helps explain the number of older long term comic book readers we've seen try more Vertigo titles, Dynamite's pulp/adventure type characters, and in the last year especially I see these guys giving more attention to Image Comics than ever before!