If they contacted you, then you need to set the 
ground rules right away. What I usually do is 
have them send the cel first. Then, I send them 
mine. There are other way to do it. If it is a 
cash trade deal, have them send you the cash part 
of it(as a deposit) if you have a paypal account, 
then you can both send you cels at the same time. 
It is moments like this where eBay comes in 
handy. Ask to check that persons feedback and see 
if they have a reliable track record of 
completing deals. It's a good way to get some 
quick background on someone before dealing with 
them.