Last time I bought a s/h mtb I met the seller at a motorway service station. It was £1800, I looked it over, nothing was broken everything was as described. I didn’t bother riding it around the car park, what would have been the point ? Same thing with the last thing I sold £2000 road bike. The buyer knew what it was, didn’t need to mess about riding it a few hundred yards, just checked it over and paid me the money 😀 . I wouldn’t refuse a potential buyer a test ride, but any bike I sell is an enthusiasts bike and have only had decent people showing up. If you can’t judge people at least a little bit, and show a bit of trust perhaps private sales aren’t for you.
For what it’s worth in my experience if someone insists on riding your 160mm f/s mtb or carbon fibre top end road bike around the block they aren’t going to buy it anyway.