The only consistent thing among my trades is that, 
yes, each person pays their own postage.

If you are trading some really expensive items, 
there are indeed online escrow services available 
that can act as a middleman (for a small fee).

As for when to send the items, that depends on 
your comfort level.  If someone is a regular 
contributor to an online community, I will send 
items and trades before payment is sent.  I 
haven't been burned yet.  But if someone is 
unknown to you, take precautions.  As a public 
figure on another board, I have sometimes asked 
new folks to send me items first, since I have a 
verifiable history of fair deals and references.

It's all up to you.  Good luck.

JP