Well i have not been on here long (come back to a old account from years ago) but I have noticed a big problem with people being scammed. On a motorbike website I use they use a very simple rule which seems to work. Whatever you are selling you need to upload a picture (which 90% of people will do anyway) and next to the pictures of the thing for sale you need a piece of paper with you'r username and current date wrote on it.
Also I have noticed alot of sites ask for a land line number (which is kept by mods) and only given out when there is clearly a problem and someone has lost money/parts. I know not everyone has a landline so maybe have a few options e.g link to a active facebook page, mobile number, address etc etc.
Nothing is ever going to be 100% fool proof but I think this would narrow it down ten fold and the picture idea is a very simple thing to do, If someone can not be bothered to take a few photos then chances are they are not going to be bothered about wrapping or sending you'r new product.
It's just an quick idea.
Harry