Check the history on the seller too. There's a lot of new sellers on gunbroker as on TGO. So, apply the same judgement as you would here.
The private seller comment is valid - some private sellers are hard to deal with so some FFL's don't deal with them at all. I've had some pains in the past. So, I don't like dealing with them either unless it's something I really want.
Otherwise, payment is likely by Money Order for private seller or by credit card for business. Some charge extra (3-4%) to use a credit card and should have it listed in the sale description but some don't. So, try to verify how to pay, shipping amount, and overall condition before the sale.
As for the FFL, the other comments apply. I use email to contact my FFL and have him send his information to the seller. They swap info and I have the seller send me confirmation numbers for shipping. After that, it's just like picking up something locally - fill out the paperwork and pay the transfer fee.