I've bought five guitars on eBay and been satisfied with all the transactions. I agree with Bobert emphatically on a couple of his points:

1. Check the feedback! I even try to read between the lines and I'm extremely cautious about buying from a seller with no feedback at all.

2. Snipe! I even use Bidnapper, an online sniping service, to place my bid for me as close to the end of the auction as possible. This practice makes me determine ahead of time what I'm willing to pay for the item and not get caught up in the bidding flurry that so often occurs at the end of an auction. If I lose the item because I wasn't willing to bid enough, oh well.

One thing I've done with fairly good luck is ask for a 48 hour appraisal period. You have to do this before you bid, but, if you have pretty good feedback as a buyer, the seller is often willing to allow this on the basis that you're a serious bidder who is likely to recognize the worth of the item. Whenever I negotiate an appraisal I stipulate that I'll pay shipping both ways if the item is returned. If the seller is unwilling, most of the time I'll just walk away from the auction without bidding.