I'm not sure if someone had already thought of this, but why not have the capability of demoing plugins. When you commit to a plug-in then you also commit to the limitations of that plugin, which I doubt you could write in a single product description and explain in a single video. I suggest a way to demo plug-ins so that we can see if this particular plugin works for our purposes.
You could then bar developers from packaging their project if they haven't paid for that particular plug-in. You could prompt them to pay for the total amount of all of your plug-ins before being able to package, like a sort of cart/checkout system. Or at least put a giant watermark in the middle of the screen if they have plugins they haven't paid for yet, but package their game anyway. That way we can test our games post-package or something.

The point is countless times have I run in to the problem of plug-ins having a specific limitation, in both Unreal Engine and Unity, that I was not aware of beforehand, and you're not always going to get a response when you need to know whether or not a particular limitation exists which is the most important factor in deciding whether or not to buy a plug-in. As of now it's a buy and hope for the best and a response from the developer kind of thing, which can be frustrating.