Right now, nothing is selling, but I'm also only specifically selling nylon-string guitars.
I use a combination of Reverb, Sweetwater GX, The Gear Page, the Acoustic Guitar Forum, FB Marketplace and dedicated FB sell groups (typically nothing from those except some "likes"). I'm sure there are other gear forums out there.
The best protection comes from a local cash sale. Next is Reverb. I've done a ton of transactions on TGP over the years with very few problems.
Also, never forget that Pirate Ship is your friend. I've saved a good bit using them over Reverb's shipping, even with insurance. So for instance, you have a shipping quote based on their system, and actually wind up saving a few bucks using Pirate Ship, which simply helps to offset their fees.