This happens reverse too. Domainer lists buy it now price on sedo, you start transaction, and the domain owner refuses to sell. I am talking about cases where they still own the name. If sedo posted a list of those domainers , it will outdo any list of deadbeat buyers.
The problem is most domain owners rarely get requests to buy their domains, when it finally happens they become scared and worried that if they say the wrong thing the buyer will vanish.
REMEMBER they want your domain at least the moment they contacted you, reply to their email, include your phone number, pay info etc. demand a deposit or complete payment right away or cease communications, SERIOUSLY stop communicating, the more you talk, the sweeter you are the bigger chances they feel the name is no big deal and there is no one else interested and you lose.
Ebay years ago implemented a system whereas if you agree to buy you must pay on the spot or the listing doesnt get removed.
Domain buying is sometimes an emotional thing for the buyer, something in your domain name triggered the right emotions and feelings in their brain. Emotions come and go, make sure you get your money quick, and stop being scared of demanding payment right now as in this very minute. You are selling a domain here, not a home or car with 84k miles.
PS if you are too scared to be so direct in your emails, try the following:
Once you get an email create right away an ebay auction with the asking price and make the setting buy not - buyer must pay right away
Reply to the buyer and tell them you just happen to have listed it on ebay this way everyone is protected, send them a link and explain its a buy now so no need for bidding etc.
If they take no action they where not serious in the first place, you where dealing with a buyer who simply had a brain fart and is making your entire day stink because of it.