I’m author of a popular yet complicated plugin. It’s complex by nature, I mean it may cause few incompatibilities with other complex or non-standard plugins. In some cases fixing these kind of incompatibilities are hard and sometimes technically impossible.
A buyer truly expect from support to get a working product, from other side I can’t spend days to make it work with any plugin.
What is my legal responsibilities? What is your suggestion?
Actually not quite so… If you advertise your plugin as a standalone plugin you are not responsible for other plugins and their functionality. The same goes for plugins that you offer compatibility for. As those third party plugins are not a part of your product, you are not responsible for their malfunction (for any reason).
On the other hand, theme business is different. If you include a plugin in your theme, then you are responsible for everything related to that plugin.
@rayoflightt agreed. For both themes and plugins business, you should mention what is not supported. Buyers cry if they come to know about the unsupported plugins and they demand a refund.
Paste a large banner in item description and tel them to contact author before using this plugin with other plugins.