Ebay is another source of used gear. Again, it's a one on one sale so usually no guarantee, unless the seller specifies a return policy. Also, unlike Craigslist and FB Marketplace, there are buyer protections with Ebay/PayPal. Ebay tends to tilt towards protecting the buyer when a dispute arises.
If it were me, for a big ticket item like a used HF rig, I would stick with a major radio dealer like HRO, MTC, etc. You're dealing with radio people who are knowledgeable about the equipment and the dealer has a reputation to protect. Years ago, I purchased several high-end communications receivers from Universal Radio in Ohio. (now closed due to retirement). Each time, I was able to call and ask questions and the equipment was exactly as they had described it, or better. Also, it was professionally packed.