So what if somebody decides to arrange a sale on the forum and decides to send a PayPal payment and gets ripped off?
There is no measure in place to stop anybody doing it that way is there, so you would be relying on people to operate that way and you can't guarantee it won't happen, before long there will be a post on the site from somebody complaining about being ripped off having paid via PayPal
If people want to do that then obviously its there own fault if it goes pete tong, if they come crying to you then it'll be no different if another member contacts you regarding another problem i.e issue with forum etc I know its hassle on your side...
But, having it collect in person takes it out of your hands, all you need to do is have a parts (for collection/deliver) section with no threads bar one which makes you click a link which shows you the disclaimer page telling you outright in big bold writing whatever you want inc do not use paypal or contact admin with any issues, if they tick and agree then they get to view the parts page, again it will reiterate its between them and not this forum, if they still (probably will get one) uses paypal then quite simply - tough, if the person makes the travel and gets greeted with a part not as described then as no money has been handed over they turn around and head back home, feedback page/rep might help a bit here
Having a good few number of posts to be able to agree to the terms might help aswell, anyone with less will only see a link saying click me to get greeted with up your posts
If people want a part they will make the effort to travel, if its too far to make the effort then tough, someone closer gets it