A slight tug when exiting from park or neutral is normal, a violent jerk is not (unless trans is fully built e.g. TH400 full reverse manual HV pump and race clutches). On CVT transmissions it should be almost Unnoticeable. The jerking could be caused by the fluid being to thick or dirty.
When some people sell cars to stop the trans from slipping they add stop slip. This thickens the fluid and can cause that as well. If you know that is not the case, it could be that the motor mounts are weak which is making the normal tug seem more vigorous. Use commonsense.
Use a code scanner check for trans codes, if the IM monitors are complete and no codes, then ok.
If the monitors are not complete ask the seller to drive or allow you to drive it till they are complete and then rescan.
Short of checking the trans fluid pressure in all gears, the only way to tell if your getting a good deal is experience. If you know someone you trust to steer you in the right direction, take them with you and have them give the car a look over.