My rule with MC sales is: If you want to test ride, you hand me my full asking price in cash. After that I'll toss you the keys and you can ride it. Bring it back intact and I'll give you back your cash and we can talk price. Drop it, wreck it, or damage it in any way, I sign the title over to you and its yours.
In the many years I've been doing this (over 20) I've only had one potential buyer actually do that (and that was when we were selling my wife's Intruder to another female rider. She was a beginning rider and wanted her dad to ride the bike to make sure it was OK.)
Most of the time they just come, look at the bike, start the engine and then once they've verified that it's in decent shape, we start talking price.