As mall owners pour money into top-tier malls to add experiential elements and other features unavailable online, the owners of a large number of lesser-quality malls are pushing to offload the struggling assets.

Trouble is, buyers are tough to come by, especially at anything close to the prices that sellers would like, Bloomberg reports.

Sales figures in the retail sector reflect the trend. According to Real Capital Analytics, only $3B in retail real estate sold in April, down 27% from a year ago, and the lowest monthly sales in the sector in more than five years.

That reflects not only the Amazon effect, but perhaps a more fundamental division of retail into winners and losers.

"A sharp bifurcation has emerged within the shopping center sector. High-quality grocery-anchored REITs appear poised for continued growth, while lower-quality power center REITs will continue to struggle," writes Hoya Capital Real Estate, which tracks REITs.

Prices reflect that bifurcation. According to NAREIT, citing Green Street data, average cap rates for the top 100 malls are about 4%, and for the next 200, cap rates are about 5.4%. For malls ranked 300 to 500, the average is 8.4%, and for those at 500 or below, the average is 14.5%.

Part of the decline can be chalked up to the shriveling of the department store as a retail model.

"I don’t think anybody really anticipated the decline of the department store to happen as quickly as it did,” PREIT CEO Joe Coradino told Bloomberg. The company has sold 17 malls since 2013, and Coradino added that the properties wouldn't fetch as much now as only a few years ago.

Department store woes aren't the only ones bedeviling malls. A wide range of in-line retailers are downsizing or disappearing, not always with much notice.

"Given that in-line tenants have a higher rent per square foot burden and shorter lease terms, these [closure] trends will often occur long before any anchor store closing announcements," Green Street Advisors reports.