THE MOBILE FIRST INDEXING UPDATE 
As per the latest update from Google, it will now access the mobile version of the website first for indexing purpose. If there is no mobile version available then the desktop version would be considered.
BACKGROUND BEHIND THE MOBILE FIRST INDEXING
A large number of users have started accessing ecommerce websites through their mobile phones. This means that purchasing through the mobile phone is on the rise and mcommerce is likely to take over the current e-commerce scenario. 
MOBILE INDEXING AND ECOMMERCE WEBSITES 
The mobile indexing update invariably means that all the e-commerce websites would need to be mobile responsive. Earlier, a mobile responsive ecommerce website was only relevant in terms of the mcommerce factor but now its importance has increased manifold due to the mobile indexing update by Google. 
WHAT SHOULD A RETAILER DO ABOUT MOBILE INDEXING 
A retailer’s ecommerce solution provider would first need to analyze the nature of his business and check where he is getting the maximum traffic from. Despite the rising mcommerce, a retailer might be getting more traffic from desktops rather than mobile devices. In that case, the retailer need not do anything immediately but still, it is advisable to keep an eye on the emerging trend and its effect on the business. 
 Those retailers who are seeing rising traffic from mobile phones or who already have a higher number of visitors from mobiles compared to desktops need to go for ecommerce solutions that provide mobile responsive websites. 
IMPORTANT TIPS FOR RETAILERS FOR MOBILE INDEXING
If you are in the retail business and you are not too technical then you can still manage by keeping the below- mentioned tips in mind.

The retailers can deal with the m-commerce and mobile indexing factor only through a mobile responsive website. So double check those features with your retail commerce solution provider.
Speed is a very important parameter in any kind of website. If your website takes a long time to load then there is a high risk of visitors bouncing away from it. Google’s Accelerated Mobile Pages (AMP) feature helps a great deal wherein the load speed of the mobile web page is drastically higher because a toned down version of the mobile web page would just appear in an instant. But in order to enjoy the benefit of AMP, it is advised that retailers have mobile responsive website in place.
Content is another very important parameter to keep in mind while going mobile responsive. You may write and present your content in a certain way for your desktop version but the same may not work for a mobile friendly version. As a retailer, you need to be clear about what message you want to convey in order to avoid content related confusions.

RELATED QUESTIONS
         1. What is the difference between mobile indexing and mobile responsive?
Mobile indexing is an update by Google. Mobile responsive is a type of website that is optimized to work on a mobile device.
2. What will be the cost of a mobile responsive ecommerce website?
         The pricing would depend on a large number of factors related to your retail business
          3. Does no mobile site mean no indexation
Google will also index desktop websites. Priority would be given to mobile-optimized sites over a desktop one.