I'm also having this problem. I've tried via the UK site and the COM site and in both cases the resulting page for
checkout.payments.sandbox.ebay.com loads with no stylesheet and buttons that do nothing. This is extremely frustrating as I have customers waiting and there is no way to get in touch with ebay support without shelling out hundreds of dollars to report a problem with their own site. This is criminal extortion and should be illegal. If this isn't resolved soon I'm going to have to try and develop on the live site and invoice eBay for the charges incurred which I'm absolutely certain they will refuse to pay.