Might be an internal problem on the site. Have you tried contacting ebay? If they've done a software update it could be a bug, or a downed server. My wife bought some stuff off them yesterday and the payment stuff was working fine then.
I tried before years ago. I remember they just kept me on hold forever then not resolved my problem. I think I unconsciously quit eBay the first time then as I wasn’t particular angry or anything, but I guess when I thought of my buying options I had a negative association with the site. Since getting back to it I hadn’t had any other problems until today.
I’ll try a few other things and then give customer service a call. I just have a feeling it’s going to be a headache without any results.