It is possible to buy physical Farnkos with PayPal but it is otherwise difficult and/or expensive to do so for non-physical frankos, because of significant risk to the seller.
While it is possible to find an individual who wishes to sell Franko to you via Paypal, (perhaps via #franko-otc ) most exchanges do not allow funding through PayPal. This is due to repeated cases where someone pays for frankos with Paypal, receives their frankos, and then fraudulently complains to Paypal that they never received their purchase. PayPal often sides with the fraudulent buyer in this case, which means any seller needs to cover that risk with higher fees or refuse to accept PayPal altogether.
Buying Frankos from individuals this way is still possible, but requires the seller to have some trust that the buyer will not file a claim with PayPal to reverse the payment.