Rule 1: Buy and sell what you know and love.
Indulging your passion for the guitar is what makes buying fun and sharing your passion with others is what makes selling easy. If you do not love guitars and have a playing knowledge of at least one make or model of guitar on which to build, don't bother. You will be missing out on both the fun of playing lots of guitars and on any profit from the experience because there won't be any. Sorry. Unless you really love guitars, it's just not worth the inevitable aggravation, even for the most seasoned guitarist, of making a bad purchase once in a while. And if you are inexperienced, the likelihood of buying and selling smart (see Rules 3 and 5) is all that much lower.
While the number of makes, models, and vintages of guitars can be overwhelming, focus on those you already enjoy. Drop in on as many guitar stores as you can to sample their inventory and become familiar with the models you like, especially if your experience is limited to the guitar you own and/or usually play. For acoustic guitars, for example, consider what body size, neck profile, and wood type(s) you prefer. Start in the price range you would feel comfortable with as a buyer yourself. This will help you know a good deal when you see one and offer a competitive price when you are ready to sell. It will also prevent you from getting financially over-extended.
New instruments have both a published MSRP and a lower actual retail ("street") price, which may or may not be published. Dealers cannot publish prices below the guitar-maker's authorized street prices, but they may offer further discounts in person or over the phone. Be sure to ask for the best price available on the models you are interested in so you know what the real market value is. Dealers take trade-ins all the time as well so ask to see any used models they have for sale.
Used instruments are generally available well below their original sales price and this is where bargains can be found. For used and vintage instrument values I start with the Blue Book of Acoustic Guitars. This will be the best $20 you spend. It will help put any further information you glean online or in a shop into proper perspective. (Note that there is a companion volume available for electric guitars).
The best online resource for actual market prices is, you guessed it, eBay. Search for any guitar model and chances are it's listed. Now go to the advanced search function and select "completed listings" to view recent sales for a real-time snapshot of current market value. (Note that the condition of the instrument and the quality of the listing and seller will greatly influence any given sales price). While you are at it, you can save your search and choose to have new listings that match it emailed to you daily.
Rule 3: Buy smart.
This is the most important rule. There is no profit unless you really do get a bargain with a wide enough cushion to allow for any additional expenses related to fixing, adjusting, and preparing the instrument for re-sale with a set of new strings. Besides researching the qualities of a potential model, you need to become proficient at appraising condition, playability, and tonal quality, which will determine value. The more experience you have playing and assessing different guitars (see Rule 2), the better you'll become at avoiding duds.
Keep in mind that a guitar can be worthless (except for parts), for example, if the neck is warped. A poorly maintained guitar can have separating parts, cracks in the wood, and crazing in the finish. Dings, scratches, and an impregnated smell of smoke will also reduce value. If needed, a neck reset, which involves steaming off the glued neck from the body to reset it at a proper angle, will set you back several hundred dollars. You need to also be aware that any changes/repairs or even "improvements" to vintage instruments can actually crater their resale value. Educate yourself on the basics of guitar maintenance so you can spot these issues and determine their effect on value.
As always, if it seems too good to be true, it probably is.
Rule 4: Establish a working relationship with a good luthier.
Rare is the used guitar that does not need some adjustment and some TLC to bring it back to optimum playability. A good cleaning and new strings will do wonders for resale value. You can also easily learn to check for proper string height and do simple truss rod adjustments yourself to achieve the best intonation possible. For any necessary repairs though, you'll need to enlist the help of an experienced luthier who can advise you, as a poorly repaired guitar will be hard to sell at any price. Fellow guitarist enthusiasts may have a recommendation or you can try Craigslist to see if there are any freelance luthiers in your area as their rates will likely be well below those charged by your local guitar store. Don't give them a major repair job until they have proved their skill and trustworthiness with lesser projects.
Rule 5: Sell smart.
Craigslist is a good place to start. The advantage is the transaction is face-to-face and in cash with no transaction costs. I recommend a neutral public place so you do not have to let a complete stranger into your home. I rent a nearby office/storage space that has worked well for this purpose, but I have also sold a guitar in an office parking lot. Bringing the guitar to the buyer's local workplace has also worked well for me. Be flexible, but be safe. Price should be set near what you really want for the guitar. Generally, the more attractive the price, the likelier you are to get any response. Expect to negotiate based on the buyer's in-hand evaluation of the guitar, but don't do this until you are face-to-face and you've already convinced the buyer he really wants it. Also, be aware that Craigslist is rife with email scams. Be sure to use Craigslist's email randomization feature to withhold your actual email from your listing and do not respond to such vague and cookie-cutter inquiries asking "Is this guitar still available," which are designed to harvest your email if you respond. Experienced Craigslist users include in their listings specific instructions for inquiry emails to distinguish legitimate missives.
So there you have it. There are now many valuable online resources, both free and paying, that expand on any number of issues flagged here. If you do take the plunge, know that you will be providing a valuable service for your fellow guitar enthusiasts, while expanding your own enjoyment of the guitar. And who knows, you may even come out ahead with a few extra bucks for "just one more guitar."