No matter how many pics are posted from my perspective there isn't any way of knowing 100% that what you're getting is what you expect it to be. All that gives you is some basic info about the condition from cosmetic standpoint. There's a lot that may show up in photos.
So it all comes down to how much do you trust the seller and do they stand behind the guitar they're selling. Have they been honest about it's appearance and playing condition and can you return it if it's been misrepresented. If so then just like buying anything else online you're fairly well protected especially if you're using PayPal to process the transaction.
I think some may want to avoid buying extremely expensive or rare guitars this way unless they're familiar with the seller and are comfortable that the guitar is not a "fake". I've purchased a couple of used guitars from online sources and the experiences were good. I also bought one from a TDPRI member and that experience was also a very good one. I'd search this and other membership sites first to see if what I was looking for could be bought this way. It's often your safest bet.
I think the important thing to remember is that it isn't a brand new instrument and it will in all likelihood have a few flaws that you may not have expected. It's virtually impossible to cover everything in an add. If you aren't prepared to be at least somewhat flexible then buying used online is probably not for you. You're better off to see, feel, and touch first.
CS Nocaster; "Nashcaster"/Nocasters; MP Thinline/P90s; Nashville/Onamacs; CVC/Keystones; Lenti Esquire/ Lion, CV JBass/J45s, CV PBass/P46, Taylor 414ce, Breedlove C250/CMe, Roland Cube 40xl, Roland BC 60/CRex; Bugera v5/"Chemp"; tc BG250 Bass Combo.