I always ask how many previous owners and why are they selling. I also list this info when I sell - not that I buy and sell very often.
How many owners tells me how many times it's been shipped around. CA to NY to TX to wherever else probably takes a toll. If the unit has been passed around, it raises red flags. Maybe there's something wrong with it, but not outright? Does the 3rd owner really know the unit's history? I ask for serial numbers and ask the manufacturer, but that's not fool proof either.
As to why selling, maybe there's something wrong with the gear. That's my way of kind of getting in writing that nothing's wrong with it. I also ask if anything is wrong with it, but the more ways I ask, the easier it can be for someone untruthful to slip up.
I have no problem with it didn't sound right in my system, I bought it to try it, upgraded, etc.
If someone doesn't want to answer an honest question, I'm done with it. It doesn't take very long to answer how many owners and why selling.
I see how those questions can get frustrating with tire kickers asking a million questions and not buying though.