There are likely some specific issues with the 150 and hopefully some owners will kick in.
But in general, when buying a used outboard, do the following...
Pull the cover and look for corrosion, errant wires, or anything else that's out of place.
Compression & leak down test. Compression should be at least 200 psi on each cylinder and there should be no more than a 10% difference between the highest and lowest.
Pull dipstick and if oil has not been changed recently, smell if for any oder of gasoline. Make sure it is not milky, which is a sign that water has gotten into the oil.
Pull magnetic drain plug on lower unit and check for any metal flakes. A little metal like talcum powder is OK. Make sure that the oil is not cloudy.
Check the prop for deformations and major dings on the blade edges.
Check the tilt/trim mechanism for any significant gurgling or being excessively slow. When fully raised, check the T/T hydraulic tubes for leaks or seeps.
Do an on-water test if at all possible (if not get a warranty for at least 30 days after the engine is mounted on your boat and can be water tested.) Water test should be through sustained ranges in gear from trolling speed to wide open throttle (WOT) and everything in between. Transition speeds should be smooth and no sign of misfiring or hesitation.
While testing on the water, check the stream of water from the pisser - it should be luke warn or a bit hotter like bath water.
After testing on the water, pull and inspect the plugs. They should have a light brown coating. If any are blackened, you are running too rich.
After testing on water, smell the oil again for any oder of gasoline and check for any milkiness.