It is all about ROI, return of investment. That is what he cares about, and so should you.
You should be interested in the products he wants to sell. Most e-commerce websites are talking about "datafeeds", you should learn what that is.
You need to understand all the nuances of selling a product, shipping, taxes, etc ... so you can show this data to users coming to your website. Take a look at creating shops on Shopify, maybe you can even use that instead of building a website.
You might also want to learn about adding products to Amazon Marketplace and Google Product Search (google product search is a price comparison website, basically). Amazon can sell products for you, without you even having a website. And Google Product Search, and other price comparison websites, can drive traffic to your products by making them appear in search results.
Money clearing and companies that handle that (clearinghouses) are important too, but you should care about it only if you want to actually deal with them directly. Usually there are middle-tier companies that provide you APIs for this sort of thing.
The above are only suggestions and ideas you might use for asking your client questions, there are probably more things I didn't cover.