Duplicate content issues is the biggest problem for most shopping websites. The problem comes with manufacturer description, most owners of shopping sites simply copy and paste product descriptions which was provided by supplier, manufactures, or host of affiliate network. Word to word description appears on multiple sites across the web and cause collective punishment from search engines.
Here how Google, Yahoo, and Bing work with content… their algorithms is created to find original source of unique content and avoid appearance of a websites which steal content and violate copyright laws. When 1000’s shopping networks and sites use the same manufacturer description it moves all of them into omitted results. Often only one site remain in Google, Yahoo, or Bing index as content source.
Example:

Description from Amazon and Target websites for wooden hangers appears in 172 other websites and networks. As result, Google index only 13 of them and moved other 159 networks into omitted results with 0 chance to appear in search index when people search for wooden hangers, cheap wooden hangers, affordable wooden hangers, large wooden hangers ets…
Read more to find how to avoid duplicate content