First, let me say that nothing beats walking into a well-stocked premium guitar store where the staff have been playing, maintaining, and selling guitars for years. Whether you are a beginner or an experienced guitar aficionado, such visits should be savored not for the prospect of buying a guitar, but for the pleasure of experimenting with the visual, tactile, and auditory cornucopia of multiple instruments one after another. Comparison playing is experiential learning at its best and the differences are often not so subtle as to be beyond even a beginner's appreciation. How you rank those differences however, is subjective, so that there is no definitive list to consult as to the best guitar at any given price point. Such a visit should thus be part of every curious guitar player's repertoire whether you are in the market for a guitar or not.
Once you have identified the size, type, and perhaps brand models that you most enjoy, and gotten over the sticker shock, you can buy a new guitar from the store with which you are by now familiar. This is probably the best bet for the beginner or intermediate player with little guitar-purchasing experience. In the unlikely event you purchase a lemon, you can enlist the store's help in activating your warranty remedies (a repair or replacement).
For the more experienced guitar purchaser there are other less expensive options online. Local listings on Craigslist allow you to test drive the principally second-hand instruments and negotiate a good price directly with the seller based on your assessment. This holds true even more with premium models, where you want to make sure the guitar has the sonic and playing personality you desire before dropping several thousand dollars. The downside to a local Craigslist search or even a guitar store is that certain brands and models are rarely listed or on display. Even a well-established brand like Guild, for example, has little representation of its current model line in the Boston area and only occasional local second-hand listings.
The obvious alternative is an eBay search, opening up a huge national market for both new and second-hand instruments. If no private or commercial seller has the particular guitar you are looking for at the time, you can have daily search results sent to your email so you don't miss a listing.
purchased a 2001 model earlier this year off of Craigslist locally. It has a cutaway grand concert-style body, solid Spruce top, Rosewood back and sides, scalloped bracing, Mahogany neck with bound Rosewood fingerboard and pearloid block inlays, and logo and Chesterfield pearl inlays on the headstock with Gold Grover tuners. After a professional set-up and some cosmetic repair work, it is one of the nicest instruments I own.
The particular listing title for this 1999 model was incomplete ("Guild F-47CE"), so I was pleased but not surprised to be the winning bidder at $400+ under the Blue Book value for this top-of-the-line (at the time) F-47 model. I was even happier upon delivery of a nearly flawless guitar as described. The original Guild hardshell guitar case is in like-new condition. The guitar itself has no dings and only some minor pick wear on the sound-hole above the pick guard to suggest that it has been well-played over the last dozen years by its single owner. There is one small area of binding separation from the body, which seems to be a recurring issue with these Westerly-made Guilds, but an easy fix. The frets were professionally replaced and are like-new, and the nut was upgraded to bone.
The disadvantage of eBay is that you are purchasing a musical instrument without being able to play it before delivery. Some sellers are happy to give you an in-hand description over the phone, but this is only as good as the seller's ability to give you an accurate read on the instrument. Some also post video samples of the instrument being played, which can be helpful. I only had a brief written description and some pictures to go on. This is why I held my breath as I took the Guild out of its case for the first time, tuned it up, and played it... Joy and relief! It has a balanced warm tone that is well-defined across all registers whether amplified or unplugged. The sustain is wonderfully rich and strong. The action may be a tad high, but that is easily adjusted. A week later and having enjoyed it over the Labor Day weekend, I am still reveling in my good fortune.