I've traded with people here. My recommendation would be:
Only trade things that you wouldn't mind losing (to start out).
Don't trust anything that seems too good to be true.
If you're selling, make sure confirmed addresses are involved.
Lots of people on here have Ebay accounts as well - check feedback there
Mainly though, don't try to turn a profit on here. When you're involved in fair trading, most people here have been surprisingly honest (I'm not a big internet-truster and am always pleasantly surprised). If you're trying to pull a profit, you've jumped into the pool where the sharks like to swim and are more likely to get bit.
So yeah, to answer your question, there is no guarantee. Another thing I would recommend is communication with the person you're trading with. You'll get a feel for the type of person they are through talking with them. At the end of the day though, you're dealing with people, and there are no guarantees when dealing with people.