75% of my collection has been bought online most of the sight unseen. I have bought used and new. When buying used I buy from know sources. Places like CDNN, J&G, Dans Ammo etc... People I have worked with in the past or people on Gunbroker, Guns America or Auction Arms who have lots of positive feedback.
The key to growning a collection and getting the most for your money is a good transfer dealer who will not only transfer gun to you but will transfer/ship guns for you. My guy does both for $10 and charges me what USPS charges him for priority mail, insurance & delivery confirmation.
Pics are always important. A phone conversation always happens before the $$$ is sent. Any negative vibes and I walk away from the deal. Better to have to battle negative feedback then loose money.
I also buy off the classifieds on fourms like this one, THR, Sig Forum, Colt forum etc.... There the rep of the seller is obvious. Some of these end up being face to face deals but many of them have ended up being FFL transfers.
I have been burned only once and I did not loose money I simply got a gun which was not as advertised and had to be repaired. In the end I sold it to another individual with full disclosure because the deal left a bad taste in my mouth.
Being smart and buying from multiple sources including online can save you thousands over buying local only. There are some guns I would only purchase in person. Lower end 1911 are one of them. The fit and finish of the guns vary from one to another often based on who put them together. For example if I was looking at a RIA tactical I would go to a store. Ask to see 5 of them. I would open the boxes and inspect each one and pick the best of the group assuming one meets my criteria. As others have pointed out they can and often will be differences in one gun to the next.
The other option is to develop a relationship with the seller so that he or she will pick the best one for you. I buy a lot of stuff from Dan and Dan's ammo and he will hand pick a gun for me as a personal favor. He picked out my Izzy BHP and I could not be happier with his choice....
Even when buying oline relationships matter. My good friend Wild Alaska who used to post here is my Colt guy. He is also my BHP smith. I have bought tons of stuff from him over the years I have yet to meet him face to face but if I need a NIB Colt he is the one I call. He picks a nice one out of the stock charges my card with no fee and sends it to my FFL !!!! In returen he gets a lot of my custom work for BHPs and 1911s.
All that said if I am looking at something NIB I will often ask my local guy what he can get it at. If he is within $15 to $20 I will simply order it from him. Often on more common NIB stuff he will be close enough that he gets the deal. If not he is happy to do the transfer which does not add to his overhead and is a pure profit deal for him...
In this relationship everyone is a winner!
This is the modern world of buying guns now. More and more dealers understand that the gun market like all markets is online and national no longer local monopolies. I personally think it is only smart to take advantage of this larger market. I would not have been able to buy my small collection if I was restricted to my local market. YMMV