In general, I think it probably is safe right from square one. It's like a community here and people want to stay a part of it... Why poison the well by cheating on a deal?
Check out the Marketplace Feedback area here and on other mobile nations sites to see what people's reputations are. Look at their post counts, number of likes and thanks received, read some of their posts to get an idea of who they are...
Look at other Marketplace threads to see how active they are in making swaps rather than buying/selling... If they do a lot of trades or have listed an item you want to trade for before you even ask, these are all good indicators.
I will say worldspy99, Bunky, pankaj981 and ajst222 have all come to my attention as frequent and honorable swappers just from me keeping an eye on what goes on in Marketplace. (LOVE this forum, even when I'm not buying or selling I read it quite regularly.) I'm sure there are more members that deserve a mention, but those 4 stand out as "frequent fliers" in terms of trading and volume of activity.