Read and understand all the rules. Do not deviate from them.
When posting an item for sale, be as descriptive as possible. It's all well and good to say stuff like "the pictures show the condition" or stuff like that, but a lot of times, the photos don't tell the whole story because the photos are not very good. Don't post a photo and expect prospective buyers to guess whether it's a 6-inch or a 6.5-inch barrel. And if numbers match, say so.
Pictures! You're allowed five pictures as attachments. If you use a photo hosting site, you can post as many photos as you need by using a direct link.