I'll answer my own questions based on my own investigation.
GPL2 states:
If identifiable sections of that work are not derived from theProgram, and can be reasonably considered independent and separateworks in themselves, then this License, and its terms, do not apply tothose sections when you distribute them as separate works.
From my understanding, a plugin that I write myself and I sell as a standalone deliverable is a separate work, thus not bound to GPL2.