What are the advantages of buying a new guitar vis-à-vis a pre-owned instrument?
• Buying a guitar in a reputable shop has the following advantages:
++ Your new guitar will be just that – brand-new and untarnished (unless you go for a relic’ed axe).
++ The shop assistant can help you finding the right guitar for your playing style.
++ Many shops also offer to set up your new guitar to your own specifications.
++ Should there be any problem due to faulty workmanship, you’re covered by a warranty.
• Why can it make sense to buy a used guitar?
++ In many cases, you will be able to afford a slightly more expensive instrument. Depending on the age and condition of the guitar, a used guitar usually goes for about 60-80 % of its price when new.
Please note: This kind of price reduction tends not to apply to collectable (vintage) guitars, as well as many special run and/or custom shop instruments from sought-after brands. The value of some of these instruments may even rise considerably with the passing of time.
++ A pre-owned guitar is also pre-aged, so the possibility of nasty surprises due to wood shrinkage/expansion is also considerably lower. If a three- or five-year old guitar’s neck is still straight and its fingerboard hasn’t shrunk, chances are that things will remain stable in the future, too.
Shopping for a used instrument does require some experience from the buyer, though, because he/she will have to be able to correctly gauge the condition and market value of the instrument on offer. If you feel unsure take a more experienced guitarist along with you.
• If you want to buy a used guitar via the internet, you should make sure to apply the same amount of care, as with other shopping on the net. Don’t hand over any credit card details to a private seller, and direct payment in advance is also generally a no-no. The safest way to do business is by using well-known platforms, such as Ebay or Reverb.com, which provide for relatively secure payment options, such as Pay Pal.
Remember there are also pirate copies of popular guitar models sold on the internet. Again, if you feel unsure, as a friend for additional advice. And remember the old adage: ”If it’s too good to be true, it probably is!”
Kitarablogi.com – Finland's premier Guitar and Bass blog 
Check for cracks or repair lines as headstocks frequently break. Verify that each tuner is reliable and operates without a hitch. Every electronic control needs to be checked. Look for internal bracing separation, degradation, or body cracks if the guitar is an acoustic. Check to make sure the neck is securely fastened to the body and doesn’t seem to have been fixed! One must have a general understanding of how guitar parts function before making a purchase.