well I dont trade cels much but when I do, its 
usually we both send it at the same time, and we 
both insure it for an agreed amount on both the 
cels (and pay for our own postage). I don't know 
about having a middleman, because that can get 
complicated. Usually if you don't know the 
person, you can ask others on the forum or you 
can check their ebay feedbacks.  

I hope your trade is successful =)