By a special regulation of the European Union, as of February 15, 2016, disputes related to online purchases throughout the EU will be possible to resolve through the ODR platform, which you can access here.
This means that if you encounter an issue during an online purchase within the EU (such as a faulty product, inability to exchange a product, etc.), you can submit your complaint in a quicker and simpler way at the link above.
Both consumers and traders can use the platform, and complaints can be filed in any of the 23 official languages of the EU.