There isnt one as they can be misused by people with evil intentions.
Take money for game and not post it then in a week or so say you did it must of been lost in post.
(Have had this problem on another site)
I think the idea was toyed with when the site started, but was taken down as its to much hassle.
Best stick with Ebay/Craglist
But at the end of the day it is all down to the admin