As a seller sometimes, I hope to be treated in a nice as manner as I treat the customers (pretty good actually).
Things happen and situations do occur beyond the control of buyer or seller.
What you could do in the future is to actually read the feedback statements (if you hadn't before), also how he responded and acted during any previous questionable situations, which tells you how he would treat you if a situation of some sort came about.
How he words his auction description can be a clue also. There should, in my opinion, be a respectful tone, and a desire evident to help the buyer through the sale and treat him as best can be in order to bring him back and buy again.
Hard to tell sometimes. You have to make the call yourself at what you feel comfortable with, money-wise and time-wise and communications-wise.
Also, a 100% seller may be only 100% this year. Look back a ways too and see what happened before. See if he improved or stayed the same or got worse in his customer relations. Things change over time.
Hopefully the bad sellers get weeded out by lack of sales or bad feedback.
For you to know when to give bad feedback is a judgement call. Try to take all the conditions into account. The length of time an item usually takes to travel, holidays there and where you are, strikes, weather, fires, roving bands of rancid chickens, etc, etc.
Look to see how long he usually takes to deliver. Some sellers ship once a week or at least not every day. Sometimes customs slows things up if international.
Even after all that is considered there still may be things you do not know about. Like sick children. In that case it is up to you how long you wait. Sometimes, depending on the payment method, you can start a claim and then if the item arrives, cancel it and everything is OK. This might make some sellers upset but heck, they are there to follow through on their promises as best they can. Or want to.
I had a surprise operation while selling and my brother stepped in and sent emails to all waiting buyers telling them I would be out soon and most were very understanding, but they could see there that I was attempting to communicate my good intentions and deliver as best I could.
I could go on at length about this as I have been stung at different times and learned the hard way to be extra careful and remember what happened to me and others and pay attention to all the details.
If this guy was OK to star with, a sick child really and for truly changes how he prioritizes things I would think. Or if he is faking that (that would be pretty bad indeed) I hope he gets what is coming to him. No need of such behavior.
Partial rant over

.
edit typos