Best Buy shares are down 14% today after posting disappointing quarterly results.

As CNBC contributor Herb Greenberg notes, the real story is how Best Buy's online sales growth is collapsing.

In Q3, Best Buy reported online sales growth of 7% year-over-year. But that's down sharply from 16% year-over-year growth in Q2 and 26% year-over-year growth in Q1.

The winner here is Amazon, which is likely taking share.

Now read: 10 HUGE Questions About Apple's iPad 2