The best diagnosis is to consider what you, yourself, would prefer to see when buying a similar firearm.
As a general rule, more photos are better. Using balanced light (indirect sunlight) is a good idea, too.
Spell checking and diction matter, too. This is a cruel reality of modern times. Our texting & emailing generations generally have left grammar and diction at the Old Folks Home, but, they matter. Getting a third party to read your ad copy is a very smart thing to do, as it can be the difference between a sale and no sale.
Having you payment and shipping options spelled out clearly is also a good thing. Also, if you are not offering things other sellers offer, having a good explanation can matter, too. Simple things, "My bank is a long drive, and only accepts [payment type}." can make the difference. You want the ad to read like one you'd buy from.