I have sold / purchased about 25 firearms on line in recent years, so a few thoughts-
Seller feed back is important, and a good indicator of the type of person you are dealing with.
Numerous SHAP, CLEAR, DETAILED pictures is important. I wont even
take the time to deal with someone who cant be bothered to advertise a gun with nothing but a poorly focused cell phone picture.
Buyer / seller communications. If a seller refuses to talk to you on the phone, to discuss any relevant questions or concerns before a sale, then walk away.
A "no return" policy makes sense for the private seller. To many people have buyers remorse when buying expensive items on line, and have the "I'll just send it back" Walmart type mentality. Personally, I will accept a return, but it's pending my inspection for use / damage, and the understanding that the buyer pays in advance the insured return shipping costs, as well as any FFL fees I will incur prior to my refunding his money.
All the above can really help ensure a smooth transaction, though there was this one guy I sold a 1940's Winchester 71 in .348 Win., that I talked to numerous times on the phone with prior to sale, making it clear exactly what he was getting. Still left me a poor feedback because even though the rifle was "great", there was no owners manual......
Larry