In the context of the COVID-19 pandemic, online sales have increased in recent years for many products. Responsible consumption has also been considered by households and individuals, and interest in sustainable products has positively evolved. Although sustainable products have more and more been considered by consumers and producers as appropriate alternatives, the results are still underwhelming. This study aims to demonstrate the relationship between the online sales of sustainable products and the online marketing costs expressed by the cost-per-click in Romania in the last three years. This quantitative research is a conclusive and descriptive study based on secondary data sets collected from the evidence registered in an online agency, which, in turn, was based on a sample of companies and products in three industries considered highly open to sustainable approaches: electronics; home and garden; clothing and footwear. The results show different relationships between the online marketing cost and the sales of sustainable products in the above-mentioned industries. In addition, online consumer purchasing intention is a mediator for the relationship between marketing cost and online sales in one industry only, namely electronics. The study reveals the development of the online transactions of sustainable products, considering the connection between marketing costs and subsequent sales.