May only need an imu calibration, but I wouldn't buy until he did one and you saw it flying correctly. If the cal doesn't fix it you would be into changing boards. Could ramp up the cost quickly. Depending on the deal, you might be better off looking into a refurbed system. Practically everyone who has posted here about getting a refurb has had a positive experience. They really are 'good as new' and come with full warranty.
Does this mean sometimes flying sideways without stick input or hovering in wind from one side. In the absence of a significant weight distribution issue it will hover level in still conditions.
If IMU has been done on a levelled surface, I really don't bother the aircraft's levelling during flights. I'm more concerned if the camera captures tilted horizon videos.