I have never faced a situation like this one in my career. Because I always buy after researching about that particular stuff. Not only for themes and plugins, I do proper researching of every product or service. That's why I don't need to worry about the themes or plugins, that they will work or not? Because I already knew that it's going to work fine.
However, when I bought something without researching about it and got some issues in it. The first step I will do is contact the developer. If the developer isn't responding which is in some rare cases. I will start posting reviews about their service online on different forums and sites. This will make him respond to my request fast or the service he/she is offering will go down forever.
Moreover, I have never done this because never faced any issues with my products.