Take it from me!  You can get a great deal on a guitar by buying it on eBay.  As an amateur musician, I've been buying and selling guitars for 40 years, and I've purchased three of them on eBay.  Each purchase has been more than satisfactory and I got each of them at great discounts.  What are the tricks?  Well, there are only two: (1) know what you want; and (2) be patient.

Knowing What You Want

Do not buy guitars on eBay unless you know exactly what you want beforehand.  Knowing what you want means not only knowing the brand and model of the guitar you want, but also knowing the average price in guitar stores (Sam Ash, Guitar Center), in want ads (Recycler, Craigslist, etc), and on eBay ("completed items").  Make sure you know the exact specifications and accessories  that are included in these prices.  Sometimes what seems like a good deal really isn't; it comes with a gig bag instead of a hard case, it has inferior pickups to the one in the store, etc.  Remember, when you're shopping on eBay, you're shopping by photos and the seller's description and reputation.  Ask the seller questions about what you don't know or can't see from the description or photos and what is important to you.   Before you commit to a bid, make sure that all your questions have been answered or ones that you are prepared to live with unanswered.  Knowing what you want also is knowing what weaknesses certain guitars have and whether that weakness is showing up.  Knowing what you want is knowing what kinds of flaws you are willing to tolerate or fix in search of a fantastic deal.  Knowing what you want is knowing all of this BEFORE YOU BID. 

BEING PATIENT

If you've ever shopped on eBay you are familiar with auction fever.  Auction fever is when someone loses control of their senses and wallet in an effort to win an auction, no matter what the cost.  I have seen people pay 50% more for a guitar on eBay than what they could get at a retail store.  I've seen bidding wars for guitars that ended up selling for way more than they were worth.  But I've also seen some incredible deals go down, people literally stealing a great guitar for unheard of prices.  I know, I've done them.  What's the difference in approach?  First, remember that you don't shop on eBay to pay more than retail; you shop on eBay to get good deals.  That means knowing what you want and what price would be a fantastic deal.  Knowing what you want is knowing what you're flexible enough to do without (hard case, color, pristine finish) in order to get that special price.  A word about finish: unless one is a collector, a few nicks and scratches or a gouge or two should never dissuade you from getting a great deal.  Any guitar that has had any use at all has a flawed finish.  Expect to spend a year or more waiting for your guitar and price.  That doesn't mean it will take that long, but it might.  Stake your price and stick to it.  Don't get caught in auction fever.  Believe me: You WILL get your guitar at YOUR price, and that feels realllllly good.   Sign up for email alerts on saved searches and each time new items are listed go and check them out.  I check twice a day.  You can snag a Buy It Now superdeal from a guy who needs moolah fast.  On auctions, don't commit too early; hang back and watch from afar.  If you enter a bid, someone with auction fever will jump to outbid you.  Better he should think the auction has little competition.  Then get yourself an online bidding site--there are several--that will submit your bid at the last moment, a process called sniping, a rather harsh name for a time proven auction tactic used by sophisticated bidders in the most genteel auctions around the world.  The good thing about these sites are that you load in your top price and commit to a final bid.  No chance for auction fever here! 

WINNING AND LOSING 

If you are doing this right, you will lose far more auctions than you win.  Being a smart buyer means getting the great deal.  Sure that was a bitchin' Strat and if "you'd only bid $25 more you'd have won it."  That's a sucker's regret.  The smart buyer knows what he's willing to pay. The smart buyer says, "Sure that was a bitchin' Strat, but the guy who bought it payed $25 too much."  Wait it out, pounce like a lion in the bush.  If it's meant to be, you'll end up with a good guitar AND a great deal.