I have bought several used guns on-line and benefitted. I agree with the positive statements above and Art Eatman's key points.
I would only add that
a) Pay with a USPS postal money order and send the check by postal mail. You get, theoretically, at least, the full resources of the Postmaster General in investigating and prosecuting Mail Fraud, a federal crime, if you file a complaint against a seller who totally rips you off. Make copies or scan the check and counter receipt along with a printout of what you think you're buying, the item identifier (for ex., Gun Broker item #), the sellers photo/words about it, etc. Then you have some proof of your side of the the transaction.
b) Unless you ARE an FFL, you're going to have to receive the firearm through your local FFL, pay a transfer fee and pass a background check. Ask the folks at the gun shop/FFL to handle the gun, have a look at it, ask "what do you think?", etc. Most of these folks are fine with spending half a minute checking it out and telling you what they think. At some shops, you could pay for a real inspection, with bore light, partial tear-down, etc. (but I never have).
c) I agree with talking with each seller on the phone, but also writing some E-mails. See how responsive (timeliness) and willing they are. Mention their seller rating (most forums and auction sites have some sort of "community" rating system) in a friendly way so they know you have checked it out and are going to give them a rating.
Generally you benefit. I recommend non-FTF buy/sell/trade of used guns. But there is no escaping that there is an element of trust that must be extended and received between buyer and seller. Thank God you live in a society where this is generally working well as a rule rather than the exception. See Francis Fukuyama's book Trust for why/how this works and what happens if it doesn't.
I suppose there are rigorous methods by which you could remove all doubt. But that would cost more that the firearm and/or spook away the honest, good sellers/buyers. I would not buy/sell from/to someone who made me jump through a bunch of hoops...