Further to this Guide: http://reviews.ebay.co.uk/SELLERS-BEWARE-BUYER-PAYPAL-POSTAL-SCAM_W0QQugidZ10000000003706754
It doesn't even help if you have proof of postage! It doesn't even help if two sellers are affected by the same buyer. It doesn't help if you get positive feedback. It doesn't help if the Postage label is purchased through PayPal. No-one cares if you made a charitable donation as part of the sale.
PayPal and the Banks collude to get their money from you thereby negating the impact of section 75...
My recommendation: don't use PayPal. Personal cheques only for me from now on.