There should be a rule, if you deadbeat one time, you get a warning. deadbeat 2 times, you are perm banned.
Though that might be the thing to do in certain good faith circles, it doesn't make sense.
I tell you, that it's bad when you waste time negotiating, and then the buyer leaves you high and dry and sometimes even fails to respond.
But what's worse is when you turn another buyer down, because our deadbeat-wannabe-buyer-scumbag has assured that he can pay you more.
In a scenario like that, if your on Sedo you may lose the deal altogether, and still be stuck with the name, you almost sold!
So a deadbeat is a deadbeat, and ban should be instant.
If your on a forum, maybe a fraction seems more appropriate.
Honestly speaking having huge lists of dead beat buyers will kill the cause, and multiple websites with huge lists might not be the answer. Maybe a central database yes, but not many sites----with huge lists isn't going to make a difference IMO