Your location isn't listed on your user title, so I'll just offer this general warning: in some states, even a good online deal can be offset by the ridiculous transfer/processing prices that you will have to pay and the effort you'll have to go through.
For example, I'm in MD which isn't a very gun-friendly state. I got a good deal online, but then with the fees I had to pay it didn't turn out to be nearly as good as I originally thought. I still made out ahead in the end, but just barely, and there was, of course, extra hassle involved. I think they purposefully want to the make the process (well any firearm purchasing process) a PITA here.
If you live in a free state, it should be easy and you will most likely get a better deal than locally.
You can get a better deal online but asking a dealer to accept a firearm from out of state that he has in stock (at a higher price) is asking to be gouged by the transfer fee. I had a dealer tell me "you'll pay through the nose" if I bought anything online that he had in stock.
Generally, if I buy a gun online, it's because its a vintage model or otherwise hard to find locally. Shipping and transfer fees easily add $50 to the price (xfer is $30 around here) so for most new, current production guns, it's not worth it.
Just a suggestion. On many milsurp rifles, you can get a Curio & Relic FFL (relatively easy to obtain) and have long guns sent direct to you if they qualify as C&R firearms. Many milsurps on the market do.
I was given some advice not too long ago to be very careful buying used guns online; especially handguns. Oftentimes these dealers will sell range guns that have had thousands of rounds pumped through them. I don't know...I guess I would rather spend a little extra for the peace of mind that will come from buying new or from someone I trust.
Instead of speculating; just go to a few dealers locally and ask what their fees for transfer and processing the ffl/background check are as well as any other fees that might be attached. Once you get the quote, go online and see if the fees still make the effort worthwhile. It doesn't hurt to see what the charge would be for a store to order something not in stock as well.
I have no problem buying online. I do have a C&R FFL but I will often buy firearms not on the list. My dealer does not charge me for accepting them, he knows if he does I will take all of my in shop business elsewhere as I do buy a considerable amount of primers, powder, etc from him as well as firearms.
When it comes to quality it is simple, do not buy from any dealer that does not offer an inspection period.
Did that before the conversation ever took place! The guy was in the process of buying the shop from the previous owner who'd handle anyransfer on for $20.
Funny thing was I was asking about ordering a stripped AR lower that I was planning to build. His response was, "you won't save any money building one." To which I replied, "no but at least I'll know that I built it exactly like I wanted it."
New firearms don't worry me as much as long as the warranty card is present, if there is a problem the factory will take care of it but yes, even new gun should still have an inspection period. I have sold a number of firearms on gunbroker.com and have always allowed and inspection period.
AuctionArms.com is another good site, as well as GunBroker.com. eBay won't sell guns, etc., but these sites are set up the same as eBay, which makes it pretty easy to purchase, and usually, not always, allows an inspection/return if the item shipped doesn't meet expectations.
Get to know your local FFL. Look on gunboards for FFLs who offer discounts/good prices. I used to pay $30 per transfer. Now I go through a guy set up in his own home for $10 per, 3 for $20. Very handy when buying 3 stripped receivers at a time.