Not sure what you mean by black market but I would look at ebay 'open box' phones. I bought my v40 as an open box and it was brand new with a heavy discount.

 

My fingerprint reader stops working occasionally too. I've found that giving the power button a short press to lock the phone and again to unlock it keeps me on the same screen and gets the fingerprint scanner working again. It happens less since the last Verizon update.
Randomly restarting I haven't had and can't think of anything that would cause it. I would think, if you called support, they'd say to do a factory reset. I hate doing that but they do work occasionally. What to do depends on how much you like the phone. I like it, especially for the earphone jack & sound, so I would try the factory reset as a last resort. I just read that LG is releasing a bunch of new phones within the first quarter of 2021. They're doing different things with their screens, if you can wait. Good luck.
BTW: I'm not big on used phones, even refurbished. I've never had much luck with them and seem to inherit other's problems.

 

I don't use the fingerprint scanner, so I can't be much help. Outside of the reset, best I can suggest is to do a simple restart. (My wife keeps hers on 24-7). I have mine set to restart every Sunday at 2:00AM. That helps clean out cache items.

 

Sounds like a bad phone or possibly one that was dropped many times. I've used phones from a few different manufacturers and never had one without issues. I'm used to finding work-a-rounds. It all depends on how much you can tolerate. It may be time to replace it. If you do, I'd look for the LG V60 on sale if you can't wait. Before getting any phone, check out the forums it has and see the kinds of issues people are having. Then you'll know what to expect.