Landlords have malls they'd like to unload. But who'd want to buy them?

As they battle the rise of e-commerce, U.S. mall owners are trying to clear their books of fading centers so they can focus on the most-profitable ones. That's proving difficult, with just a shallow pool of investors who are willing to take on a declining mall and even fewer who would pay what the landlords want. Only about $3 billion of retail real estate changed hands in April, a 27 percent drop from a year earlier and the lowest monthly tally since February 2013, according to the latest data from Real Capital Analytics Inc.

Mall giants such as Simon Property Group Inc. and GGP Inc. are spending billions to update their centers, adding experiences that can't be found online and reinventing the cavernous spaces left behind by failing department stores. But there's a growing set of lower-tier malls that have slid too far toward irrelevance to be worth a costly overhaul.