Purpose: This study aims to identify the core competency of sellers on their sales performance in the context of e-commerce, and to verify the mediating effect of innovative sales activities. Research design, data, methodology: A questionnaire survey was carried out to 200 sellers in e-commerce. Exploratory factor analysis and reliability analysis for Cronbach's Alpha were performed to verify the validity and reliability of the and the regression analyses of 5,000 bootstrap samples were carried out to test hypotheses. Results: Based on the analysis, the elements of competencies of sellers in e-commerce such as self-control, trust-building, social interaction skills were positively related to the innovative sales activities and sales performances. Innovative sales activities also affected sales performances. Indirect effects of competencies of sellers on sales performance via innovative sales activities were partially effective depending on variables that brought practical implications. From a practical perspective, the valid competencies found in this study may be applied for the recruitment and selection of sellers. Conclusion: This study shed light on theories of e-commerce sales and sales practices by examining the competencies of sellers in e-commerce compared to those of traditional sellers and by revealing innovative sales activities as the effective mediator for sales performance.