I had this same issue but then luckily got more money than I thought, my choice was new D80 or 2nd hand D200.
Basically this was going to be a choice of 2 almost identical camera but the more expensive one had better weather protection etc. In the end I got a new D200 from Hong Kong but having seen both cameras I am glad I got the better one.
Basically you will get a better camera going for
second hand and get all the advantages or such. The disadvantages are:
Wear
This is easy to see even in photographs. The rubber goes smooth and levers loose their coatings. Doors might become looser as might other things.
Unseen damage
It is possible that damage could have been caused which might be expensive to repair such as using a 400v flash and blowing the flash board inside the camera - This would not be visible. Bent pins on the memory card slot - easy to spot it tested at all. Bent pins on expansion slots - USB, video, power, remote etc - these need checked. Out of alignment - It is quite possible that a bang on the camera might put the focussing system out. The screen on the back is small enough that you cannot easily see focus so again you need to check. Focus assist light - Yes I have seen this.
Consumables.
Batteries - These wear out, new ones are new!
Guarantee
A new camera will come with the manufacturers warranty. If the camera fails in (often 2 years now!!) the warranty period they will fix whatever it is and the camera should come back as good as new. If the seller goes bust the manufacturer will still normally sort this out.
Freebies
Often camera manufacturers will give freebies with new cameras. We have had free grips, free SB900 flashes, money off vouchers and even cash back