Surely if they just pay by paypal (not gift!) and you keep a record of posting it then nothing can possibly go wrong?
Not if you follow all of PayPals regulations and requirements, no.
But it is still a massive ballache as PayPal sides with the buyer initially. Ringfencing the money. It is then down to the seller to prove he has sold the goods and posted etc.
It's such a ballache to prove that most sellers give up and write off the cost - which is how the scammers make most of their money - people just giving in.
At the end of the day you have to weight up how much you want the money VS associated risk.
There's absolutly nothing wrong with replying to the potential buyer saying
"Unfortunately you are a new user and I am not willing to sell to users with no posting history - try to build up some activity on the forums and engage with the community before using the classifieds."
In fact quite a few services and forums I use already force such a system where a user has to have X amount of activity/rating before they can post in some sections.
But then again, if you have 3 very wide eyed hungry children to feed then maybe its worth the risk - you have to decide yourself how much the money means to you and how quickly you need it.