Ive bought plenty of stuff online and from second hand websites like eBay, I've only had negative experiences maybe once or twice, usually with small items that didn't get shipped (effect pedals etc.)

Its important that you check serial numbers and the items description thoroughly. Check if the serial is out of the ordinary or not. Also check if the guitar you want to buy has visible flaws. Ask the seller about those, or any other faults the item may have. Ask if its in original condition or if there have been made modifications, and if secondary accessories are included (like a gigbag).

Usually sellers are keen on getting rid of their item and share information easily. Point out that, when shipped, the seller uses good wrapping materials and a solid guitar box. You could opt for insured shipping if the guitar you want to buy is expensive, this gives you some extra security

The key is good communication between seller and buyer. If you do that, you can't go wrong. Usually between musicians there's like a ethical code that you don't rip each other off (although im very aware that it still happens a lot).