Have you even tried selling items on Facebook ?
Never seemed to get a response by a personal message or someone says they have PM'd you but it never arrived in your inbox.
This is pretty common and there are hidden messages in your inbox, if like me you dont normally fully open your mailbox just use the drop down menu and click the new message then check this out.
Click on the Drop down then onto show all at the bottom on the drop down.
You will notice that there is the word Other next to inbox, Click on this and you will see all the messages that are from people who are not in your friends list and probably people who have said they have PM'd you on your selling sites.
Once you have opened this for the first time it will then appear next to the word inbox on the drop down menu.