My advice is that before you attempt to purchase any firearm on line, you arrange for a dealer in your area to handle the transfer, as online sales must go through a dealer to be legal. You will not find a reputable seller that will send you any firearm directly unless you have your own federal dealers license. Everything you need to know about purchasing firearms is at F-Troop's website. If someone is willing to send you a firearm directly without you having a license, it is probably a Bloomberg sting, and you can pretty much count on spending the next few years at Club Fed. Pack lots of KY jelly.
That being said, there are several established websites on which you can conduct transactions and that have dealers listed who are willing to handle the transfer for a small fee.
Something else to consider is what an individual FFL will charge for the transfer fee. One local purveyor of fine firearms charges $35, while another pawn shop just 10 minutes away will charge $20. Two guesses who I go to.
Another web site to check out is GunsAmerica.com. They recently revised the software running their operation, and it seems to have settled down to actually giving you what you were searching for.
On just about any of the brand-specific sites (GlockTalk, CZForum, CZShooters, and SIGforum to name a few) there is a classifieds section for buy/sell/trade of both firearms and accessories. Only the firearms (except for .22lr conversion units, which have no receiver) need to go through an FFL for transfer, unless you can do it as a FTF(face-to-face) sale.