Research, and trust. Check out the other trader's feedback score, look at their profile and read some of their posts to get a sense of how they talk & act, and decide whether you want to trust them to ship when you two agree to ship. An "educated gut instinct" I guess is what I'm saying.
I'd also recommend sticking with a trusted site like Bladeforums. Buying, selling, or trading off of social media and auction sites can lead to some... disappointments. That can and does happen here, too, but not as often. And you have more resources available here to help prevent it or to help sort it out if it does happen.