First of all. I'm not doing trades. I joined, because I want to help the site growing, but I still know about trading stuff etc. Done it some few times before.
When you're dealing with a person, then it's hard to tell, if they are trustable or not. Some peoples are really good at SE (Social Social Engineering). Specially when it's over the internet. You have no real contact to the person, so you can't tell how he look or if he is suspicious.
It's hard to prevent scamming and it can happen from anybody. When you're doing trades, then you either have to trust the other person or not. How can you do it somehow? Look at reputation vouches and feedback of the person in previous trades. Remember it does not always count, peoples still do it even they have a high reputation etc. Reputation does not mean a person is trustable, neither does anything else, but the reputation can tell feedback from others, just like the vouche can.
Scamming happens everywhere, on the internet or in real life.
I guess the best way to prevent it, is actually not doing any trades, but other than that, then there is really no way other than you either trust or do not trust the person you're dealing with. Remember to look at previous trades, feedback and the way the person is talking to you. If he wants you to go first and you're not 100% sure that you will receive the thing you're going to have money/the thing you bough, then do not deal with them at all. You have to be 100% sure that you will receive it, when you're buying/trading/selling.
Other than that,
goodluck to everyone