I have found both Flippa.com & their Flippa Escrow that is PromisePay very prompt and effective in their customer service.
Though this matter relates to the buyer submitting the funds to Flippa, nothing could be done if the buyer does not pay or fails to honor his commitment, only contacting him through the Flippa Sales Page whether he is interested about the domain or not will be a good approach.
Even if he is a new buyer it would not be fair to evaluate him just because of it, I have made 2 sales on Flippa & both were new buyers, they paid immediately.
Summing up! Best of Luck, hopefully the buyer will honor his commitment.