As a workaround, you could Contact the seller and ask him to send you a Paypal invoice directly, including the eBay transaction number.
That should be viewable by eBay and in any case you would be covered by Paypal's solid Buyer Protection program.
When you pay, the seller should mark the purchase Paid manually which covers you with eBay's Money Back Guarantee.
If you want a third level of protection, back your PP account with a credit card, for their chargeback policy.
I have had a fairly busy weekend but no customers have told me they have had problems paying.
Since two different browsers had similar problems have you tried Old Faithful? Toss cookies, close computer, go for a nice walk, and come back to try again?
