This is the response I just got from Auctiva;
Thank you for contacting customer support. I'm sorry to hear that you are having this issue. We are not receiving reports of this nature that indicate we are having a widespread issue, it is more likely that there has been an error between the checkout and winning bidder's system. To complete this transaction it will be necessary to arrange for an alternate mode of payment. I would recommend the 'Request Money' function in PayPal. Assuming you both have a PayPal account, this would be the fastest, easiest solution. We apologize for any inconveniences this issue may have caused. If there is anything further I can do to assist you, please don't hesitate to ask!
IMHO (ok, not so humble opinion) This response is entirely unacceptable. It places the burden on my customer instead of on Auctiva where it belongs. I have disabled Auctiva Checkout for all future listings. Unfortunately, I can't disable it on my current listings without ending and re-listing them all....