1. To lessen in number as a result of being sold. Our stock is selling down, so be sure to grab your copy of this limited-edition item before they're all gone!
2. To reduce the number one has of something by selling it. In this usage, a noun or pronoun can be used between "sell" and "down." The company's founder has begun selling down his stake in the business, which has caused many investors to grow nervous.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.