I haven't used coinpurse.io myself so don't know all the details but the yway you are describing it it sounds like a Ebay sort of site where you are purchasing through their platform but dealing with other members of the public and not coinpurse.io directly.

This means that you won't be able to sue coinpurse.io if they haven't done anything wrong as you were dealing with another member on their site. All you can do is contact their support and hope that the member is banned from using their platform.