Topic creator requests mediator, mediator sends you a PM. You fill up what is traded and who is trading what.
Before you give the item, fg is sent to mediator and he sends you PM that he has the fg. You give the item to the trader and after you both confirm that the trade went OK with mediator he sends you the fg.
So, no. They do not safeguard your item nor do they join the game to hold your hand. They hold the fg hostage ensuring that you will in fact get paid for your item after the trade.
Same rules still apply (as they will tell you in the PM exchange) take screenshots of the trade in (rare) case you need evidence that you did in fact give the item.
Free tip: Don't trust strangers on the internet that they are who they claim they are on jsp without confirming via jsp....