As another member suggested, use PayPal.  Have the
buyer send the full payment upfront (it's standard
for this community), but on your end you have to
ship your cel with tracking.  I don't know if you
live in the US or outside, but you will definitely
need tracking.

For very expensive items, double-check PayPal's
seller protection policy.  I heard that sometimes
you can better benefit from signature confirmation
and insurance on really expensive items.

Most people around here have a fair reputation,
though.  Just take a couple steps to protect
yourself in the process.  :)