A Basic Checklist
You can save some money by purchasing a used guitar. You can also end up buying a big problem.
There are some things you should consider if you are trying to buy an acoustic that has been previously owned.
1. Start with the neck
Is the neck angle warped? Is the neck twisted? Is it cracked? If you see any of these signs, then chances are the guitar has issues which could end up costing you a bundle.
2. Check the bridge
Check for cracks in the pinholes. This sort of problem can easily be repaired. But if there is a problem with the bridge plate, such as a non-original plate. This should be a red flag. If the bridge has lifted and been improperly repaired or ignored, then the guitar may suffer a fatal problem. Also check for epoxy on older guitars. This should be a no-buy situation because it will (not may) will ruin the guitar. I personally would never buy a guitar that doesn't have its original bridge plate.
3. Look at the braces
Are the braces loose? It's not a deal breaker but you should expect to spend money to have the braces repaired. Tap the guitar body and if you hear something flapping around, then you know you have loose braces. Proceed with caution.
4. Look for obvious cracks in the wood
Older guitars may develop cracks. If the cracks had been repaired properly and is cleated in, then you can still buy the guitar but it should cause you to pay a little less.
5. Inspect frets
Are the frets worn? Do they need fretwork? This is something that will cost you money to fix so reduce your offer accordingly.
6. Look for refinished guitars
If a vintage guitar has been refinished, that almost certainly will devalue it. But it also may have been done to cover up damage. Inspect refinished guitars very, very carefully. Personally, I avoid them because there's just no reason to accept the risk that comes with these instruments.
7. Check the guitar's original origin
If you're buying a rare, vintage guitar, be sure to ask about its history. Try to track previous owners and documentation that may yield secrets you'd otherwise uncover. Once you are satisfied that you have researched the instrument and verified it's authenticity, you can proceed.
If you buy your used guitars from reputable dealers or people whom are known to you, then most often you will avoid getting a nasty surprise after you've brought your new (used) guitar home. Even then, people make honest mistakes and it's always best to run through this little check list to make sure you know what you are buying.
You may want to try visiting Reverb.com if you're looking for a good used guitar. They have lots of resources for musicians buying and selling instruments. Sweetwater.com also has a used gear marketplace worth checking out.