The Mercent E-Commerce Performance Index (MEPI) is a report that measures year-over-year growth of the same-seller gross merchandise value (GMV) for national brand name retailers selling across E-Commerce sales channels. And for the second quarter of this year, an increase of 17% over second quarter last year was reported.Mercent clients that sell through Amazon's marketplace program saw sales grow 35% during the second quarter.

Clients selling through all Google retail sales channels, including Product Search, Product Listing Ads and the Google Affiliate Network, grew 21%.Based on analysis of current and historical MEPI data, Merchant predicts that the 2011 holiday season will deliver solid double-digit figures; however, that might be slightly weaker than those posted last year.