The real question is probably more like "How do I go about picking a boat for me?"
Well..... for me, it's actually closer to the real question for a couple of reasons....
I sail in fresh water great lakes, so the punishment of the salt sea is not an issue. The #1 reason of a used boats condition seems to be original build quality. You can really see it looking at a ODay, compared to say, a Pearson.
I realize that previous owner care counts for alot. But, in big freshwater lakes, no-one is going crazy outfitting their boat with lots of stuff to go long distance cruising. Most boats are pretty minimal for equipment and this is less of a factor in used boat cost.
This comes down to there being a host of used boats in the 1970-1990 range all around 27-36 feet long (most have stayed in the lakes their whole lives). The three factors that seem to determine the used price are (in no particular order)
3. Original build quality
There seems to be consistency in #3. You walk in some brands and its all wood finish, and the build quality is obvious. Others, not so much. I'll make the intellectual leap that the quality in those certain brands extends from what I can see to what I can't.
To take a more mundane example:
I bet you can tell which end is the better quality