Thanks for all your replies. I already looked at both the US and Dutch Paypal user agreement terms.
There are differences, but only in wording really. For example the Dutch terms bluntly state that the seller is not protected in case of sales outside ebay (11.6/11.10). The US regulations state (11.5): "Items/transactions not eligible for PayPal Seller protection: PayPal Direct Payments." I take this to be all transactions from one paypal account to another not made through a buy-it-now button like on ebay. So to answer my own question in the OP: the seller is NOT protected in outside-ebay, private transactions, such as on APUG.
But, the same goes for the buyer side of things. Dutch (13.3) and US terms (13.3) are identically unequivocal on this: "Payments for the following are not eligible for reimbursement under PayPal Purchase Protection: (13.) PayPal Direct Payments".
So as far as paypal is concerned everybody is on their own once outside the realm of ebay (or most other business, non-private transactions): no protection for either seller or buyer. But then who gets suckered when nonetheless the buyer opens a dispute? According to the above terms the buyer should not get his money back.
Moving on, where do the credit card companies join in? That's in US 13.7 (same text in Dutch version). This section contains the following nasty paragraph: "Credit card Chargeback rights, if they apply, are broader than PayPalís protection programs. Chargebacks may be filed more than 45 Days after the payment, may cover unsatisfactory items even if they do not qualify as SNAD, and may cover intangible items. You may pursue a Dispute/Claim with PayPal, or you may contact your credit card company and pursue your Chargeback rights."
This basically undermines all of paypal's terms, because who doesn't have their paypal account coupled to their credit card? As far as I know credit card companies, especially in the US (less so in Netherlands perhaps), have lots of power to charge back transactions made by their clients, on the basis of very little proof of item not arrived/ not as described (please correct me if i'm wrong). I can't imagine paypal being able to keep the card company's money. If that's true then so much for paypal's own rules and regulations.
I think I understand the paypal terms pretty well now. It's the credit card company's potential involvement that I don't yet grasp. Are these companies omnipotent towards the poor lonesome private seller?
Did anybody here on APUG ever overrule paypal through his credit card company in case of INR/SNAD? Or become 'victim' of the reverse situation?