I have purchased 5 acoustic guitars on ebay. Generally speaking all of my purchases have been successful and I have no real complaints. I have noticed a couple of things to look out for that I thought I might share. The obvious is that many times sellers don't disclose every problem a guitar may have. I bought one off a guy who neglected to mention that bracing was comming loose inside the guitar and that the frets and fretboard were heavily dented and spooned. This was early on in my adventures on ebay. I learned that the seller will leave it up to you to "ask" specifics and then they will normally answer honestly. Some sellers do disclose every little thing and even make some small things out to be more than they are because they don't want returns or bad feedback. Look out for the guys that buy and sell like crazy, over and over again just to make a quick buck.

The second point of contention is dating of a guitar. Generally speaking, the older vintage guitars fetch a higher dollar. Therefore, look out for the liberal interpretation of the dating process by many sellers. Older guitar companies such as Gibson and Guild have less than exact tracking explanations for serial number research unlike the Martin company which seems to have always kept it pretty easy to figure out a date of production. It seems that sellers always lean towards giving their guitar for sale an earlier date whenever possible. If there is any grey area where a date can be construed as being older, they will favor that take on the date. This is not to say they are lying, but just that they leave it up to you to be more careful than they are. I bought a Gibson J50 from a guy who said it was a 1965 model. I ran the serial numbers and it clearly fell within the range of either a 1966 or a 1969 (thats how whacked Gibson was with serial numbers - some they used over again). The guy then said "oh yeah you are right it is a 1966". Well remember I said it could have been a 1966 OR a 1969. Gibson began placing bridges "belly down" in 1969 and they were still "belly up" in 1966. This one had the bridge "belly down", making it actually a 1969. So here we have it - he started at 1965, then 1966, and finally 1969. I buy guitars to play, so it really was no big deal, but if the dates matter to you then research it to the full extent. I have seen this dating "misconception" numerous times. I have never had a seller say his guitar was newer than it was and I don't believe this to be an accident. Whenever the dating can be simply a mistake, but not an outright lie, then it is safe for the seller to be "liberal" with the advertised date. 

Bottom line, research and research some more. I love ebay and I have had a good experience, but I have also learned a lot about buying and being careful. I want to say that most sellers are great people and they are super helpful and honest. A good percentage of sellers are like I am, just guitar people who are selling because they need money or want another guitar and have to fund it before buying a new one.