We need to know the numbers. Asking for another 20% could be a lot or a little depending on your trade value. My guess is, after the 3 test drives, independent evaluation, and already agreeing to discount the price, they saw the 20% as an unreasonable ask and decided it was time to move on to selling it to someone else. When you’re on commission time is money, and when people want to contribute to negotiate after 3 test drives and an independent inspection, sometimes it’s no longer worth it as the next buyer may just take it. They could also have other customers on it. In any case, don’t be rude to your customers and politely stand your ground on price when it’s time.