Reselling online is one type of e-commerce. Let me provide you simple definitions.
E-commerce refers to the buying and selling of goods and services online (via the internet).
Reselling refers to one type of e-commerce which involves buying in bulk of goods from manufacturers, distributors, wholesalers and sell the products individually at a profit. This is the traditional form of retailing which can be either online or offline (like what the retail shops are doing or the direct sales agent).
Reseller refers to the person or company doing reselling.
The types of ecommerce are distinguished by their buying sources. Apart from reselling, other types include private labellng, stocklots or clearances, used items to name a few.
Reselling online is actually under the coverage of E-commerce. You could be conducting E-commerce without necessarily reselling something online. Activities like marketing or blogging and services alike are under the blanket of e-commerce