From a technical standpoint, there isn't much I can do to ensure a domain sale is finalised (payment made, and domain transferred). Basically, I don't have access to this kind of data. Relying on things like whois response data is not going to be reliable, and I don't want to take on any kind of additional costs for 3rd party services. I've always stipulated that the data is as accurate as the source provides. An auction finishes + there was a winning bidder = domain sold.