I am the proud owner of 11 Guitars varying in price from £28 to £1375.
Picking (no pun intended) the right Guitar is vital for your playing pleasure, so please read this guide and hopefully it will help.
Firstly, think what you want the Guitar for. There's no point in buying an Acoustic Guitar if you predominantly want to play Rock.
I like listening to and playing Dire Straits and Eric Clapton, so the Stratocaster was the obvious choice - more on that later. I also like to sing solo and strum an Acoustic, so I use a six string Acoustic for more this. 12 strings are great for strumming, but best not to have as your only guitar as difficult to finger pick or play a tune/solo.
If you are buying an Acoustic, solid tops are best. This means they are made of solid wood, rather than a laminate. They sound better and improve with age. Laminates are great if you're on a budget.
Each of the Guitars I have owned I have first tried to see if they fit me. For example, when I bought my round-backed Tanglewood, I was a relatively slim youngster and the rounded back sat nicely in the space where my expanded waistline now occupies.
That's part of the reason why I chose a Fender Stratocaster as my main guitar. It's slimmer than eg a Gibson Les Paul (and in my opinion it sounds better too). I was able to try both before choosing.
Also the necks on guitars are different shapes and sizes as are the bodies. Some Guitars necks join the body at the 12th fret and some have a cut out so you can reach right the the end of the fret board.
So.... set your search criteria to "Distance - nearest first" and go and see not just the Guitar, but the owner too. See a few on the same day so you can have some recollection of how they sound and compare.
The upside of buying from someone near you is that if they have miss-sold the guitar, you know where they live!!! You can collect too and save postage costs and potential transit damage.
Try the Guitar out - don't worry if you're going to embarrass yourself - think of what things you want to play beforehand and try them - try your favorite song. Try all of the knobs and settings to ensure they all work. The knobs are generally for volume and tone and switches select different pick-up combinations - do your research beforehand.
Look the Guitar over for damage - look at all areas where one piece joins another (the neck to the body, the bridge to the body, loose string adjusters) and see if there are signs of separation or movement.
Hold the Guitar horizontally and look from top to bottom and bottom to top to see if the fret board and neck are twisted. This may happen if strings are badly adjusted or if the guitar is in a damp room.
Ask when the strings were last changed and if it's been a long time, request a new set if you are the winning bidder. Check if there is a strap, a lead and a case included as these are expensive extras.
New guitars are getting cheaper and the quality is getting better, so consider a new one with a guarantee rather than a second hand one, though there are some second hand bargains to be had if you are patient.
As with all buying, don't get caught up in the excitement. Set yourself a budget and stick to it. That sudden urge to "buy it now" or over-bid can be an expensive mistake. There will always be another one.
If you are looking for e.g. a Takemine 12-string, check the web first as there may be deals elsewhere and this competition can be a good bargaining tool if you see one you like on E-bay.
Hope this helps.