I've had the seller ask me if I wanted to ride it before. More than once. Always thought that was weird.
The only time I would let a buyer ride a bike first, was if he were at my house, with his car parked here.
I've never needed to take a test ride myself. I think most of us can spot the flaws, or the quality, or whatever, of a bike we want to buy. I know the rims are bent, the seatpost is stuck, and the tires are flat. The fact that it's unridable makes the price come down.