The problem is that GoDaddy, eBay, Sedo, etc are NOT auction houses. And hence is a big reason they do little to nothing to enforce sales.
Reputation matters in domaining and so most domainers, including myself, follow through on deals ...
With that said, the OP situation appears to be edge case in which there doesn't appear to be a "meeting of the minds" due to a mistake / computer error ...
How one handles that depends ... if, hypothetically, the OP mistakenly bid $500 instead of say $100, that may be a situation in which, despite the extra cost, the buyer should follow-through anyways to maintain their reputation.
However, if the difference is significant, such as mistakenly bidding $10,000 instead of $100 that may be, depending on the buyer's budget / income, a situation in which backing out and taking a reputation hit is the only realistic, viable option.
Ron