That just happened to me as well, and after finding it was only the one seller which it seemed to be affecting, I tried a couple of experiments. The one that worked was clicking 'Add to Cart' rather then clicking the Buy-it-Now, then going to Checkout. It lets the sale go through then.
Seems as though people have been having random problems with this since 2008, and nobody can figure out why it's happening. But anyway - there's the workaround.
