yea buying a used outboard is always stressful. I remember when i bought mine used from a private seller. You can only do so many test out of the water. Thinks i checked for when i bought mine was compression equal on all cylinders, Check the lower unit gear case oil to make sure its not milky or extremely black. Check Prop Shaft play there should be a very small amount of play on the prop in and out no sideways movement and non on the shaft. Check the Skeg and prop itself make sure no damage to both and still has original paint. I would also put a set of ear muffs on and run the motor at neutral, forward and reverse make sure the motor performs without any erratic idling . Rev it up to see if you hear any backfire witch isn't good. Make sure it starts easy, that's pretty much all i can think of