Photographs. Lots of photographs. And they must be good quality.
One guy was a dealer who posted his ads on GB and on another forum. His photographs were always, literally always, out of focus, poor lighting, etc. And the guns he sold were always, again literally always, inferior to his description.
So many members on the other forum complained, that he got mad and quit. Fine. We don't need crooks and liars on gun forums.
I have bought on the net, and high quality fotos are a must.
Also, the seller must, absolutely must, have a solid reputation. None of this 0 feedback nonsense. And he must have a solid reputation as a seller. Not a buyer. I've heard of guys with a zillion positive feedback, but all as a buyer. Then the one gun they sold, they burned the buyer.
Also, if it's on gunbroker, I like to deal with a brick and mortar store. I'll deal with individuals (based on the above criteria), but I lean toward brick and mortar stores. If two sellers are selling the same thing (that I'm looking for), I'll buy from a store before an individual.