BST access based on post-count dont mean shit, thats been proved time and time again. The only real way of not being scammed is to buy direct and thats a fact. If you buy something from somebody you should already know that there is at least a chance of being scammed. People are dealt with accordingly im sure, but at the end of the day you are responsible for your own actions.
As of WJ invite, it would lessen the problem but not stop it. Paypal has the option of reversing payments, which is why its recommended.