Buying Facebook groups

Does anyone have experience buying (or selling) Facebook groups? I'm working on a business aimed at seniors (60+) and believe FB groups could be super effective for distribution.


Specifically I'm wondering:
1) Is there a smart way to find the most popular/active groups, other than aimlessly searching/clicking down rabbit holes?
2) Are there tools or hacks to gauge group engagement?
3) What's an appropriate way to value these groups?

So far, it feels a lot like buying website domains. There's no good data out there, and every situation is totally unique.