Don’t buy plugins that have no source code files;
If the author dies, or whatever, you can at least try to update the code yourself.
Before buying, verify how active is the author towards the community; If author is practically absent from participating with the Unreal community, just interested in selling his stuff and nothing more, the chances that the tool will be abandoned in the near future are very high because these people usually run away quickly after realizing their assets aren’t selling as expected.