Well I got my only negative from a buyer, so Id say the feedback deadline idea is not good.
The buyer hesitated, didn't reply to emails or answer the phone then 3 weeks later sent me payment, without the p&p costs. I stumped up for p&p just to get it done and dusted and left them a neutral saying they had poor communication. In return I got a negative saying I was a fraud.
You just have to live with the nobs unfortunately :-(