Used launch monitor buying guide - what to look for?

Considering buying a used SkyTrak or Mevo+ to save some money.

What should I look for? Any red flags?

Specific concerns:

1. How long do these last? Will a 3-year-old unit still work well?
2. Is warranty transferable?
3. Any way to verify it works before buying?
4. Fair prices for used equipment?

Where are you finding used equipment? Facebook Marketplace? eBay? Forums?

Anyone have a bad experience buying used?