Trades can be kind of tricky. There's no real way to ensure the trade is going to go through. If the person you're trading with has positive feedback, personally I'd recommend you send yours first, since he has credibility. By doing this, you yourself earn credibility for future trades.
Of course do your research on who you're trading with first. Private messaging other people he's dealt with is a good place to start. Also check out his posts to see how he fits in in the community. When it comes to people on the internet, you have to go to nearly stalker levels to get all the information you can.
Doing trades can be kind of scary at first, but the feedback system works well, and the mods are quick to deal with people who do shady business!