I’ve had many agreed upon deals die at the finish line. The buyer agreed to purchase a domain name – sometimes after a lengthy discussion – and then poof – they ghosted. Multiple reminder emails go unanswered, and the domain name is back on the market for sale.
One thing you can do – which I have done – is ask a representative from the marketplace / platform to personally connect with the buyer after the deal failed to try and get the deal closed. I am sure platforms like Dan and Sedo make multiple outreach efforts to close a deal shortly after it has been agreed upon, but following-up at a later date is also a good idea.