Hi, i'm interested in your BMW. Could I ask you a few questions?
Sure.  
Have there been any other owners besides yourself? Have any teens driven this car?
Nope, I am the only owner.  No teens have driven it.  I do let my 15 year-old-neighbor wash it for some cash every once in a while. Haha!
I like the car. But I feel like the asking price is a bit steep. Could you come down to say, 10 grand?
I could do 10 grand if you have a trade in.  Tell me about your car you want to trade.
Well, I have a 1986 Pontiac Fiero that's in good shape. Manual, with t-tops. Real 1980's chic.
Okay deal!  Your trade in plus 10K for the BMW.  Thanks!