Well, it depends on how handy you are and if your willing to do a days worth of work to save say $300-$500. They probably got the motor for say $100, and put $100 in parts into it. It should have a new impeller; new coils; new gear lube; a cleaned/rebuilt carb; new spark plugs and new hoses. For an amateur, that's a good days worth of labor. If your willing to get your hands a bit dirty, then it's certainly all work that you can do if you have very basic mechanical skills and a screw driver and socket set. Its pretty easy to find a motor of that vintage in non-running condition that would need only that minor work for $50-$150.
Now, if you are interested in buying the used one they have for sale, make sure you get a list of what has been replaced--not just checked, replaced. The parts are cheap enough an readily available that they should all be replaced at that price. The list would include basically everything I listed above (coils; plugs; impeller; hoses; carb; etc.). If it is in fairly good cosmetic condition, it could be worth the price they are asking--if it is in top mechanical shape--otherwise, I would look somewhere else. Old outboards are a dime a dozen, and even though the demand for running outboards peaks in the late spring/early summer--there are still a lot of them available.