Nick, don't worry. It's just that the whole idea of buying and selling stuff on the forum is entirely based on 'trust'. You can imagine that many message boards have to deal with spammers and trolls - well, the same thing happens here every now and then so it's generally encouraged to first build up a bit of a 'relationship' with the board members before you start talking business. That's all!
Re: PayPal, Stockerlone has had some issues with another member here (who has since disappeared from the forum) who gladly accepted money for the books he sold but never actually sent them (instead he sold them to someone else). PayPal only mediates when a dispute is filed within 45 days I think and if the seller is smart and succeeds in making false promises until day 45 has passed, you're out of luck. At least, that's what I gathered.