If they really want the trade, they will answer as many questions as they need for you to feel comfortable. DON'T feel afraid to ask a third party to ID an item either of yours or of your trading partner to verify the claims that have been made on that items specs. Higher-level characters normally do not want to tarnish their names to the dirt of trading and theft. Trade only with someone's real character, not their stink'n lvl 1 mule that can easily and non-regrettably be deleted for the sake of evidence removal. Of course, because you're combating with trust here, don't let down your guard too easily.
Thievery is just part of the roles that is natural for some players to play.
It is the dark path, the easy road to greatness the epitome of evil - and one of the most fun parts about trading. The more you trade, the better you get The more they steal the better they get. It becomes competition between the good, the bad and the ugly. Everyone gets burned their first time, but the scars will heal and they shall emerge the more cautious trader. I'm not saying to go out and start stealing.