I know no one can give a for sure answer but in general how long  does a mercury motor usually last I see a whole bunch being sold with 500 hours is this very common for them to last past 1500 hours or does it start usually causing trouble

I estimate my 1995 Mercury has around 2000 hours and still runs perfectly. 

No idea but I've owned a few over 30 Years old. If the compression is good and it was taken care of I don't worry about the hours. I don't know how many hours my 97 has but it runs great

Hard to answer as a lot of older motors don’t have hour meters on them.

i average 150-200 hrs per year, and chose a Honda and paid a premium.  Had a Yamaha been an option, i would have gone in that direction.  If a mercury was the only option, it wouldn’t have changed my mind on the purchase.  I would look to a Mercury owners group.  My brother in-law is at 2000 hrs on his and is looking to trade it while it is still worth something!!!!!!!

A lot of boats go up for sale once the motor warranty is up. Could that be why you are seeing what you are seeing? 5 years at 100 hours per is 500 hours.

I would recommend getting outboard and inboard motors serviced on a regular basis.  That way there are no surprises and they are always ready when you need them.  I bought a 1973 Mercury outboard a few years ago.  Had it inspected and serviced at a boat dealership.  It ran just fine.