just some thoughts 

$600 is pretty cheap for a decent canoe - you don't need to own it for a hundred years you know - you could buy it, use it and sell it for $400 after a year or two and no big deal - or you could buy it, and a month later find a canoe you like a lot better - buy that one and resell the first for likely what you paid for it or not much less (depending on the "market" you are in).  If it makes you feel better to get a deal, offer the guy $500 - one thing to watch for in a used canoe is if it has wood gunnels - if the canoe was kept outside, the gunnels tend to rot; check the bottom for soft spots too - indication that water may have seeped into the fabric from a deep gouge

do you still have the same first vehicle/house/bike you ever bought ?  of have you upgraded/bought newer ?  same with a canoe - lots of people have the attitude that they have to get the perfect canoe the first time and pass it down a hundred years later - not so

for many years, we tripped in a 15' Grumman -  that had plenty of room for two with gear;  my current tandem boat is a Bell Northstar 16.5'  - no problem for two with gear -- it all depends on how much or how little gear you will take