I can't imagine buying or selling most high-value items on Amazon: the buying side has already been covered in various places ( https://seliger.com/2017/01/09/tools-continued-careful-buy-a... ). I've sold cameras and lenses on Craigslist, which can have its own challenges, but never one as expensive as an A7R IV.
At least on Craigslist you can restrict the sales to meatspace. Asking to meet on the steps of a police station does a great job of filtering out scammers.
Are those local-selling apps any better in quality of transactions? Letgo, Offerup etc.?