Hi Elliot,
the thoeory is good but in practice technology moves very fast and the newer in therory, lower spec may have a better sensor or shutter mechanism or processing technology.
I am no expert in second hand cameras - hopefully someone will spot this thread and give you more infomed advice.
Some general points:
Think about why you are buying a camera - what are you in to - what sort of pictures - sports, scenery, general?
Set a budget you can comfortably afford.
Read reviews.
Take a good friend - tell him/her what your budget is and what you expect for it.
It's all too easy to spend just a little more, and a little more when you are in the shop.
Make sure the camera feels comfortable can you get to all the controls easily. It's not too big/small, heavy/light,ect.
Look for damage/ware - if it's "...as new Sir..." it should be.
Have they got all the manual, leads, software, etc.
Get a good warrentee minimum 3 months.
Check that the camera is still supported and you can get spares.
When was it last serviced?
Is the lens mount current?
Can you buy lenses from other manufactures (sigma, etc).
Can the lenses be used on later models.
Be a pain and give the camera a good work out.
Check the main functions are working.
Check the lenses are working.
Test the Auto and Manual focus.
If they say anything like - you can't try it because it's not charged or they don't have a test memory card - walk away.
It's your money - you don't have to buy they have to sell.
Most of the above also applies to a new camera as well.
Good luck
Welcome and good luck in your search for a camera.